Naruto's torment - Rooting for tickling

“Listen you guys, if we want to achieve something we have to work as a team. To work as a team means you co-operate. Sai, you can't just act alone. And you, Naruto...”
“T IS NOT MY FAULT THAT THIS GUY KEEPS PISSING ME OFF!”

Yamato sighed deeply at the sight on a furious Naruto pointing at Sai who, on the other hand, was smiling. The four shinobi were sitting in the room they rented, dressed in the white and gray clothes offered by the onsen. They were still wet from their previous bath and the blond ninja had his hair down due to the moisture.
Naruto is terribly hot-headed, I knew it. But if Sai continues badmouthing Sasuke, even Sakura will become unmanageable. We're short on time, but if it's the only way...
Yamato stood up as more insults flew about the room. “Enough now! Sai, Naruto, follow me. Sakura, I'm meeting you in ten minutes by the front door to plan our strategy.”
“But...” Naruto was about to protest, but was stopped by a menacing look from his taicho. Yamato ushered the two of them out of the building. The trio was walking barefoot on the grass.
“Mmh? We're not changing” asked the blond ninja. “There's no need for that. We are almost there”

In four minutes they had gone pretty far from the onsen. They were now in an isolated clearing near a rock wall. This place will do thought the team captain. He formed the seal and, under the students' amazed eyes, branches and vines arose from the ground.
“Wood style: Three Pillars House jutsu!”
A small mansion made out wood was standing before them. Yamato lead Sai and Naruto inside. There was only one, large room, completely unfurnished, accessible through a sliding door on one side. A square window allowed the light to enter.
“So, here is what I want you to do. You will stay here until the evening. Try to escape, and you shall expect punishment. Like it or not, you will be spending a few hours together”
“NO WAY IN HELL!” exclaimed the blond ninja pointing at Sai, but when he turned back Yamato was no longer in the room.


“Damn it!” he shouted, scratching his head with both hands. Meanwhile, Sai had sat down, leaning against a wall, and was shooting curious glances at his teammate.
“No way I am staying here with you!”
“Just shut up. You heard Yamato-san. You want me to explain what he said?”
“YOU SHUT UP! That's it, I'm fed up with your bullshit. I'm going”
“I wouldn't do it. Yamato-san is way smarter than you”
“That's why I am not going out through the door, idiot!” claimed Naruto proudly, striving over to the small square window. He pulled himself up and the upper part of his body was free. “Hell yeah! Did you see that, you stupid...” The rest of his sentence died in his throat as he met the gaze of a stern Yamato standing with his arms folded a few inches from Naruto's face. The blond ninja cringed and tried to produce a smile, failing miserably. “Ehm… I…”
The body of the captain went brown, then it warped. Wood tendrils sprouted from it, knocking Naruto back inside the room. The vines crawled around his wrists and ankles, lifting him off the ground and restraining him. He was suspended with his legs slightly apart and his arms spread upward diagonally. The shirt of the onsen opened, revealing his chest and Tsunade’s pendant. “Damn, damn, damn!” he almost chanted, pulling hard at his restraints, but they did not even loosen. Sai, with his smile still on his face, seemed pleased to see his teammate in that predicament. “I told you not to do it”
“Yeah, yeah, just get me out of here”
“Not a chance. You infringed the captain’s order. I am not helping you”
Naruto cursed at Sai and attempted to escape again, vainly. He yelped when he felt suddenly scratching his back. Shivers ran up his spine. Sai had quietly crossed the room and placed himself behind Naruto. “SaAai, what the heck are you doing?” “Well, since we are in the same team, you are bound to know info about me. I want to get sure they won’t be leaked to anyone. Besides, you are really annoying, and you think you’re tougher than you actually are. So, I am about to show you.”
Naruto yelped as the hand lightly grazed his side. “That’s gross, quit it!” he ordered, but some giggles mixed in with his words. He tried to get away from the inquiring fingers by jerking away, but the wood restraints prevented him from doing more than budging a few inches. It was as if Yamato had devised them for the specific purpose of having Naruto at Sai’s mercy. When the Root ninja’s other hand loomed over the blond’s tanned body, hell began. Naruto did his best to keep the laughter in, too focused to even question his own situation. He only had the feeling that giggling would doom him. The light touch was driving him to his limit.

“You know” Sai began “Root members have to undergo torture training. This is the first step, tickling. I had a brother who used to have fun tickling me”
A brother? the blond boy thought, but the only thing that exited his lips were some giggles. Sai continued.
“I bet you wouldn’t pass this phase. I can see you are ticklish. So, why don’t you laugh? Come on!”
With that, he really dove in Naruto’s flanks, and he exploded immediately.

“SAAAHIHIHIHI, Stop iiHIHIHT, DOON’T DOOOOOOOOOOOOOOOOOOON’T!”

Sai alternated between lightly tracing up and down Naruto’s flanks with his fingertips and immersing them in his flesh, as if to grab it. While the first technique kept the genin in a fit of giggles, the second caused him to block and fall silent, before letting out loud shrieks and bucking desperately. But that wasn’t even the prelude of what was about to come.

“Saaaaaaahihihihi ok stohohohop stooooooooooooooooooooooooooohp!”
“You are such a crybaby, what do you exactly have between your legs?”
Sai’s fake smile made quite a contrast with Naruto’s explosive laugh. What bombarded his brain was not exactly a ticklish sensation, more like an annoying itching, but he still couldn’t help trashing around. Gathering his full mental strength, he bit his lip. However, muffled sounds kept escaping his sealed mouth.
“Oh? You are trying to resist?” commented the Root member. He squeezed harder, but his captive held on. Annoyed by the lack of cooperation, he decided to change spot.
“You forced me”
When Naruto felt eight nails caressing his abs, he lost it again.

“Nooooooohohohohhoohohohoohohohohoohoohohoohohohooooooot thehehehre!”

That was already too much to handle, and it was one of his least sensitive spot. He knew because he remembered a certain training he had to endure to learn the Summoning Jutsu… but that is another story, and his brain could barely put sentences together as Sai studied his toned belly with torturous strokes.

“Ahahahahahahahahahahahahahahahahahahahahahahahah!”
“I wonder, what happens if I do this?”

As soon as one pale finger entered his navel, Naruto’s laughter shot up an octave, flabbergasting the tickler, who ceased the torture. The genin took that chance to recover a little.
“Wow. You sure are ticklish” taunted Sai, sitting between Naruto’s spread legs.
“Sai… please… no more…”
He just couldn’t take it anymore. He was being driven to insanity by his teammate.
“Oh no Naruto, this is far from over”
And he licked the inside of the blond boy’s belly-button. The jinchuuriki exploded again, thrashing around wildly, but this time there was no slowing down – Sai was going in for the kill. His soft, wet tongue explored every nook and cranny of the cavity.

“NOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHO! NOHOH NO NOHOHOHOH!”

Meanwhile, his hands started working on his abs and sides again, making the ticklee gasp and cough as tears streamed down his eyes. He could only keep his mouth open as the pouring laughter oscillated from deafening peaks to going silent.
This method went on for a good ten minutes before Sai decided to venture in northernmost regions. He gently brushed and counted his ribs, eliciting a reaction that was stronger than that obtained from teasing his abs, but still weaker than the ruckus caused by dipping in his navel. The Root shinobi mainly employed the “hard” touch, which seemed to work better, but Naruto was obviously astoundingly feather ticklish too.

“Ahahahahah… Plehehehehasehahahah…”
“No way. Too bad I have no tools on me and going out is out of the question. Things would get a lot more interesting”

For the first time since that hell began, the genin felt grateful to Yamato for having trapped the both of them together. But even that cold comfort vanished when eight dull nails entered his armpits. He didn’t try to hold in the laughter, he knew it would have been futile. He just howled, frantically turning his head from side to side, messy hair, still down, flailing about. Worn out by the previous tortures, he just let out incoherent sounds.

“AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH! AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH! AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH!”

The sweat that was moistening his body only made it easier for Sai’s fingers to slide all over his sensitive skin. Every time the dark-haired boy would jab the spot just below Naruto’s underarms, the blond would jump and tense up, as if electrocuted. However, it was the dull nails stroking his armpits with an unbearable appropriate amount of pressure what made him cry and lose the perceptions of time and space.

“AHAHAHAHAHAHAHAHAHAHAHAHAHHAHAHAHAHAHAHAHAHAH QUIT IT DAMNIT! AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH” was all he managed to force himself to roar. Sai was obviously taking great pleasure in making him suffer like that. His smile had turned into a wicked grin. Still, he was secretly impressed by the jinchuuriki’s endurance. He had never met anyone as ticklish as his teammate, and he was well aware that a normal person would have passed out long ago, not seeing the end of the first fifteen minutes of underarms prodding. Yet, more than twenty had passed, making it almost forty-five minutes of nearly unabated torment.

Now Naruto was but a giggling mess, too exhausted to beg, curse or keep his eyes open. Tears and sweat had made him thirsty, adding to his hoarse throat. He could only hope that his sadistic partner had had enough. For most of his worst spots had yet to be exploited. He bucked and yowled with resignation as, beyond his shut eyelids, Sai’s thumbs dove at the height of his waistline. The other boy kept that up for a while, finding out the genin’s hips were as ticklish as his navel. He squeezed and prodded and scratched until… Naruto’s belt’s knot loosened from all the wiggling. The onsen bathrobe was still in place, only open on his upper body and with the holes for the arms pulled towards the elbows to allow the dark-haired ninja to better tickle the blond’s underarms, but that single variation, of which the jinchuuriki was unaware, lead to the creation of an idea in Sai’s mind. An idea that was also a urge: Break him with everything you’ve got, everything you’ve been taught to use. He did not know if there was some desire other than cynicism prompting his doing. If there was one, he couldn’t see it. Yet, he realized he couldn’t help raise his arms to change target.

Azure irises were revealed as Naruto felt fingers dance around his nipples. That caused him to titter, yes, but also start, and, occasionally, gulp down and moan softly. That part was… enjoyable, if not rejuvenating. The genin was soon able to speak again.
“Whahat is… ah… thi-his?” There was solely naïve curiosity in his voice. Suddenly, without relenting his work, Sai looked Naruto in the eye and said “You are kind of cute when you laugh”. That was a catchphrase meant to increase the effect of the tickling during a mission of seduction-intel gathering. The blond was still in the midst of processing what he had heard when the rough tongue came back into action. So he giggled, gasped and moaned more as his nipples where teased one at a time. In the meanwhile, Sai’s hands were resting on Naruto’s thighs. Instead of tickling, they were caressing them, drawing circles, slowly. The genin felt a deep, odd feeling of relaxation and he leaned his head backwards, eyes closed again. There was nothing else. Without him even noticing, a lump formed under the fabric of the dress.
The Root member didn’t expect things to go that smoothly. It was just as if Naruto was trying his best to be the dream of any professional torturer. Sai stopped the activity on his upper body.

“You are just so predictable”

At that point, Naruto’s expression went from blissful to terrorized and, at the same time, he thought “Wait, there is some freaking guy licking my nipples!” but, before he had the chance to swear, he felt the skin of his thighs getting squeezed.
“Nohohoohohot aaaaahhhhahahahahahahahahahahahghahahahahahahahahahahahahaihihihihihin! Sahahahahahahaihihi! You ahahare GROHOHOHOSSSSSSSSSSAHAHAHAHAHHAHAHAH!”
“Not very clever of you to insult me, you know?

He then tried another approach, using his hands like claws, and it seemed to be more effective, because Naruto’s attempts to escape intensified. To think he was completely worn out until a couple of seconds ago. Insane stamina must be quite the curse when you are off the charts ticklish.
With his victim on the verge of a mental breakdown, he moved down to his knees, working behind and around them. The reaction was still strong, but nothing compared to before. He kept that up for a few minutes, to make sure Naruto wouldn’t pass out yet. To kill time, he also blew some raspberries in the bellybutton at the height of his face, creating a strange, tittering symphony.

“AAAAAAAAAAAAAHHHHHHH aaaaaaaaaahhhhhhh AAAH aaaaaaaaaaaaaaah AAAAAAAAAAAAAAAAAAAHHHH! OOOOOOOOOOOOOHHHH maaaaaaaaaaaaaaaHIIIIIIIIHHHHHHHHHH gooooOOOOOOOOooooorssssh!”

Finally letting up his “fun”, the Root member decided to examine the foundation of a certain observation of his. He had noticed that the more he “climbed” up his teammate’s thighs, the stronger the response was. So, he really went in for the kill. Unmerciful, he opened up Naruto’s vest.
“What the f…” the blond boy let out before stifling his exclamation. Sai was definitely kinky, but he looked so much into it that he looked completely oblivious to the ambiguity of what he was doing. And Naruto felt pretty embarrassed, too, showcasing a nearly six inches long hard-on in front of another guy. He would have blushed if he hadn't already been flushed beyond human.
As if to break the ominous silence, Sai commented:“Well, that's not a lot to work with, but... I wonder what happens when I do this?”
Thanks to his experience, he knew what was the best technique to adopt. So he employed very quick, light strokes with the tips of his fingers to scan a new area: the spots were Naruto's legs joined together with his hips.

“AAAAAAAAAAHHHHHHHHHUUUUUUUUUHHHGGGGGGGGGGGGGHHHHHAAAHAHAHAHAHAHHAHAHAHAH! MAHAHAHAHAHIHIHIHIHI NOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOHO! AHAHAHHAHAHHAHAHAHAHHA!”

The blond genin arched his back as much as he could while those lecherous appendixes slid horribly easily on his sweat-covered skin. Sai noticed that the closer he tickled to Naruto's balls, the more the boy thrashed and squirmed, half yelling, half laughing. Seeking new “emotions”, Sai chose to save that place for later and rake his fingers up and down the toned buttocks in front of him. If he adopted a hard-type approach, the boy would buck around a lot and try to stand on his feet, vainly. The feathery approach had a much more amusing outcome: Naruto would jump up and shrill, while staying completely rigid, not even trying to avoid the torture. So he picked the second method. When more than fifteen minutes expired, ignoring the victim's desperate gasps for air, he circled around his teammate’s anus. He was disappointed though, for the squawking laugh decreased slightly. Not expecting the blond boy to hear him over the noise he himself was making, he whispered to himself:“Weird. This usually drives everybody mad, or so I studied.”

YOU CALL THAT WEIRD?! DO YOU EVEN REALIZE WHAT THEHELL YOU'RE DOING?! was what crossed Naruto's mind, but his mouth produced only a rasping “YHHUHUHUHUHUH CAHAHA-CHAAHAHAHAHAHAHAHAHAHAH...”

Now, the blond was far less analytical than his cruel companion. So he didn't even realized that, during the crotch tickling, he his boner had swollen and throbbed several times. Because Naruto liked being tickled. As a child, he saw a sign of familiar affection in that gesture which he longed for with all his heart. He yearned for someone's gentle contact. Even when he grew up and made friends, that viewpoint stayed with him. When he thought of a possible relationship with Sakura, he viewed her playfully tickling him. But he had never imagined that the same action turned into torment could turn him on. Oddly enough, the Root member seemed to know this all along.
Sai raked his nails up and down the erected penis, eliciting an almost painful bang of laughter. Naruto’s throat and lungs were on fire, his diaphragm was contracting spasmodically without getting any air in. There was no pleasure, just tickly electrical shocks.

“MY GOHOHOHOHOHOHOHSHOSHOHOHOHOHOHOHHOHOHOAHAHAHAHAAH! AHAHAAHAHAHAHAHAHA! AHAHAHAAH AHA ahah ahah AHAHA! STHAHAHAHAHAHAHAHAHAHAHAHAH! SHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH! I CAHAHAHAHAHAHAHAHAH!”

There was no need to vary the technique, it was plenty effective already. And the blond boy seemed to agree.

“WAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHAHAHAHAHAH! AHAHAHAAHHAHA!”

His comrade’s fingers were gliding ever-so-lightly on his skin, but it felt as if thousands of ants were running all over his manhood and from there straight to his brain.

“Wow. In the top three we have a tie: perineum, dick and armpits”

Naruto didn't need to ask what a perineum was, because he still felt it tingling madly. He could no longer understand whether his sight was being drowned by tears or he was just too tired or suffering to force his eyelids open. However, much more was coming. The rankings were about to be turned. Noticing his ticklee was on the verge of fainting, Sai let up a bit. He didn't miss, however, that the boner persisted. It took the genin nearly one minute to stop giggling.

“Please... Sai... No more... please... Y-you had your fun... help me out... I won't tell anyone about this...”
“But of course you won't. First, because things have gotten quite awkward for you as well.”

With that, he tapped on the head of the genin's penis. His cock bulged and he moaned softly before managing to control himself. Only then he grasped it wasn't only sweat what was moistening his shaft. That was the reason why Sai had stopped tickling him there. He blushed even more, so much the natural golden suntan on his cheeks got smothered. The cruel boy resumed.

“You can blame no one but yourself for that. Second, because everyone would laugh at you for being as ticklish as a little baby”

To make the point, he scribbled his fingers in Naruto's armpits again. The boy was too weak to try and keep his lips sealed, so he cracked on the spot. His body had much less attrition at that point, and, despite his throat hurting terribly, he was soon in hysterics again.

“Or are you not?”
“AHAHAHAHAHAHAHAHAHAAHAHAHAHAHAHA OHAAHAHAHAHAHAHAOAHOHOHOO GH-GHOHOHOHOHOHOOHHOHOHOHOHD!”
“Just admit it”
“IHIHIHIAHAHAHAHHAHAHAHA... I'M NOHOHOHOHOHOHOHOHOHHOHOHOHOHOT! AHAHHAHAHAHAH!”
Naruto was too stubborn and proud to say it, not realizing that way he was making Sai's game perfectly.
“Right, you are more sensitive than a baby, is that right? Say it and you get a break”
At the same time, he increased the pace of the tickling, his hands darting up and down Naruto's armpits so fast they were almost blurry.
“NEHEHEHEHEHEHEHEHEHEHEHHEHEVEHEHE-VEHEHEHEHERAHAHAHAHA! Sh-SHHAHAHAHAHHAHAT EHEHEHEHEHEHEHEHEHEEHHEEHEHEHEHEHEHEHEHHEHEHEHEHEHEHEHEHHE! AHAHAHAHAHAAHHAAHAHAHAHAHAHAHAHAHA”
To make everything worse, the raven-haired boy blew a raspberry on the spot right under Naruto’s left pit, causing him to yelp and jolt.
“EEEEEEEEEEEEEEEEEEEEEEEEEHEHEHEHEHEHEHEHEP AHAHAHAHAHAHAHAHHAHHAHAHAHAH NOHOH NONO NOHOHOHOHOHOHOHOHO! FUHUHAHAHAHAHAHAHAHAHAHAHCHCKHAHAHAHAHAHAHAHAHHA!”
Then, he proceeded to use his tongue together with his hands on the same targets. The small, rough tongue wasn’t as bad as the raspberry, but it could skitter between his firm peck and armpit non-stop.
“AHAHAHAHAHAHAHHAHAHAHHAHA YOU… you bastAHAHAHAHAHAHAAHAHHAHAHAAHHAHAHAHAHAHAHAHAHAHA! IHIHIHIHIHIHHIHIHIHINOUHUHUHUHUHUAHAHAHAHAH!”
Sai was wondering how he used to have fun before meeting Naruto. Humiliating and torturing him was one of the best thing he could remember. Perhaps not even the sessions with his adoptive brother were that entertaining. While leaning forward, he accidentally rubbed Naruto’s twitching penis with his flank. Taken by surprise, no longer able to control himself, the blond was shocked with a wave of pleasure.
“AHAHAHAHAHAAHAHAHAHHAAH! Aaaah… HUHUHUAHAHAHAHAHAAHGHGHGHAHAHAHAHHAAHHA OHOHOHOHOHOHOHOHHKAAAAAHAHAYAHAHAH AHAHAHAAHAHHAHA I AHAHAHAHAHHAMMMAHAHAH! GEHEHEHEHET AHAHAHAWAHAHAHAHHAHAHAY FROM THEHEEHEHEHEHEHEHEHHEHEEHHEHEHEHEREHEHEHEH!”
Sai moved back with a disgusted look, but didn’t relent at all. “Do not even DARE do that… thing on me! You’re gross.”
As the Root member had forgotten – or pretended to forget - to stop, Naruto was too devastated by the tickling to even hear the reply. Only when he felt his own voice turning low because of the strain on his cords he was allowed to breathe. He wanted to curse so badly, but could not form a single word. Sai taunted him.

“You sure are a weakling. And ticklish like a schoolboy. Which takes us back to the list. Third: you wouldn’t want me to do this again to you. Lastly, Yamato-san was expecting things to end up like this, or in a similar manner.”
“WHAT?!”
“Isn’t it obvious? He knew you would attempt to escape, and was counting on my good judgment to teach you a lesson without overdoing it or injuring you. If you don’t believe me, look at your feet.”

At first, Naruto found nothing strange, but after a bit he noticed a small, flexible branch had pried all his toes back. He shuddered.

“Maybe I’m wrong, or maybe not. Anyway…”
“You’re not gonna tickle me THERE, right?”

The blond was too terrorized to see that giving off his most sensitive spot was not the best idea with a consummate torturer like Sai, but he just couldn’t bear to be tickled there. Even if he were not in that state, it would still be too much to handle. Instinctively, he wore his most defenseless, pitiful, exhausted expression. He could do nothing. And, unbeknownst to him, the wood restraints were sealing off the Kyuubi’s chakra.
If Sai was looking at his face was not because he took pity on him, but simply to follow his gaze to what could be Naruto's worst spot. His feet the Root member thought. An evil plan formed in his mind. Much to his enjoyment, the teammate bit his lower lip when he realized his mistake.

“Of course I will. And you will ask me to. Because you won't be getting any more breaks until you beg me to tickle the life out of you.”
“WHAT THE FUCK SAI, I'VE HAD ENOUGH OF YOUR GAMES”

He was cut short by his own howl. Not expecting him to hear over the firework-like laughing fit – again -, the raven-haired boy smirked.

“Wow, you never run out of energy... That's amazing. How longer will you last?”
The tips of his fingers were slowly scribbled all over his sack. Every square centimeter spoke of agony. But when Sai reached behind his balls, Naruto reached a whole new level of ticklishness.
“AHAHAHAHAHAHAHAHA – AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHAHAHAHAHHAHAAHAHHAAHAHHAHA AHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH TOOOOHOHOHO MUHAHAHAHAHACHAHAHAHAHAHAHAHHAHAHA! STHAHAHAHAHAHP STAHAHAAAAAAAAHAHAHAHAHAHAAAAAHHAHAHP!”
“ Hey, I'm just started! Well, this is not one of my favorite targets, so what do you say? Should I move on? Or better, down?”
The jinchuuriki tugged hard at his bonds, so forcibly that his wrists got bruised. But he could not even feel pain at that point. Still, the restrains didn't budge. He just kept laughing madly, barely managing to breathe.
“This is surely your worst spot so far, and you insist I stay here? I wonder, how ticklish can your feet be. That will be interesting. I only need your permission.”
Like many, many other times before, the blond genin would have loved to song a poem of insults at him, but was unable to. That tickled way too bad.

But my feet... CRAP!

“GGGAAAAHA! EEP! AHAHAHAHAHHAHAHAHAHAHAHAH! OHOHOHOHAHAHAH! NOHOEHEHEHEHEHEHEHEHEHEHEHEHEHEHEHE! AHAHAAHAHAHHAHAH!”

Sai had been using only one hand on his scrotum. Now, the other was crawling up his flank. That was but a drop in a basin, but every ounce of tickling was contributing to driving Naruto to insanity and back.
“You know the words...” Sai reminded him mockingly, but once again he refused to obey. The Root ninja was really curious to give a try to his feet, so, while keeping up the crotch torment, he increased the pressure and speed on Naruto's upper body, raking his slender figures up and down his ribs. That was instantly effective. It did not take sharp ears to note the difference in volume.

“AHAHAHAHAHAHAHHAHAHA! AAAAHAHAHAHAAHAHAH EHEHEHEHEHEHEHEHEHEEHHEEHEHEHEHEHEHEHEHHEHEHEHEHEHEHEHEHHE! PLEHEHEHEHEHEIHIHIHIHIHIHIHIHIHISE AHAHAHAHAHAHAAHAHHAHAHAAHAH!”
“Come on, just say it!” Sai blurted out impatiently. As his victim refused to answer through the giggling, he chose to take it up a good notch. He inserted his digits in Naruto's armpits, unleashing the dread of all the techniques he had learned thus far.
Naruto believed himself on the verge of dying. Neither brain nor lungs could keep up with that terrible feeling. His body seemed make up of just two immensely reactive spot, one between his legs and one under his left arm. He wasn't sure he could utter a sentence anymore. Yet, one last time, he didn't give up. His instinct, not conscious reason, as it had perished many chuckles ago, was shouting at him Hold on, you'll make it through. Do not let him change place. Squirming around was a thing of the past.

“AAAHAHAHAHAHAHAHAHAHAHAHAHAGGHGAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAH!”

Marveled at his teammate’s stubbornness, and thus craving to test his smooth, slightly tanned feet, Sai was no longer willing to wait. He flicked his tongue all over Naruto's abs, spiraling towards the center. Finally, it reached his bellybutton.
Now, Naruto would have traded his soul to plead with his torturer to get a break. To stop that unbearable torment. There could be nothing worse than that. But he couldn't. The moment the warm tongue dipped in his navel, he shrieked, but his voice dropped down while he was still leaning forward due to the shock, and abandoned him. He kept suffering in silent laughter for many, many minutes.

How tough is he? That's remarkable. Sai thought. When, after ten minutes he glanced up at Naruto's face, he realized there was no way he could speak. His victim was about to faint. His muscles hang like wet clothes, still, although he was still feeling the torture.

SHIT!

He ceased the tickling on those three spots and moved to the sides, one of the blond's least ticklish areas. And, instead of scraping, he massaged his flanks, so that the sensation was extremely mild. He needed to have him recover. That game was too funny.
It took Naruto a lot of time to begin thinking coherently again. Senses and speech had to wait. His chest started to heave normally, and his face lost some of the tomato red color it had acquired. When that happened, as if to prompt him to surrender – which actually was his real intention -, Sai squeezed his kneecaps and tickled under his knees and calves, making his legs twitch a little. He was not very sensitive there, the perfect opportunity to let him recover without stopping his fun.

“So” Sai finally inquired “Should I do that again or...?”
Giggly and exhausted, Naruto shook his head in what was probably a “no”. He could not even find the situation gross or weird, worn out as he was.
“Good.”

He let him get some rest, to ensure he wouldn't black out. That took a lot more time than anticipated, though, even with the genin's recovery speed. Eventually, enjoying the frightened glint in his captive's eyes, the Root member opted for a slow start. He lightly traced his nails on the back of his feet. Surprisingly, that was enough to make him chuckle uncontrollably. Naruto was sure that wasn't Yamato's original plan – and actually, he was right: Sai had made that story up to make him feel more powerless, as he knew the restraints on Naruto's toes were meant to prevent him from yanking his legs free if his body were to get lubricated with sweat – but his vines were indeed blocking his feet thoroughly, leaving his soles spread apart, immobilized, taut and much more... exploitable.
To further scare his victim, Sai moved to the sides of his feet, which seemed to be equally ticklish, using the same method. The laughter increased, just like the fear.

“S-Sahahahahahahahaihihihihhihihi... I'm be-begging youhuh... Dohon't...”

He was cut short when his teammate gave a swift tickle to his feet from toes to heel, as a test. Naruto shrieked. Even a touch as brief as a second could break him. That was going to be sooo fun. Sai started at his heels. Normally, that is not a very sensitive spot. But in practice, that caused Naruto to yelp and burst out laughing with renovated energy.

“GAAAHAHAHAH ahahahahahaha AHAHAHAHAHAHAHAHA eheheheheheEHEHEHeheheh!”

It would not have been so bad had Sakura been in Sai's place, quite the opposite, that would have meant heaven. And if Sasuke had been the tickler, he would have rivalry to fuel him up, and it probably would have been much more pleasant anyway. Those were Naruto's thoughts when the tickling started in the middle of his arch. That was utter hell.

“NNOOOOOOOOOOHHHHHHHHOHOHOHOHOHOHOHOHOHOHOHOHOHOHOH! NOHOHOHOHOHOHOAHAHAHHAHAHAHAHAHAHA AHAHAAHAHAHAHAHAHA AHAHAHAHAHAHAHAHAHAHHAHAHAHAHAHAHAHAHAHAHAHAHAHAHAHA! COHOHOHOHHOHOOHHOHOME OHOHOHOHOHOHHOHOHOOHHOOHHOHOHN!”

On the other hand, Sai appeared as though he was having the time of his life. Every movement of his nails wreaked electric havoc that reached the ticklee's brain and mutated into deafening, screaming laughter. Naruto started to buck like never before, going absolutely ballistic. Sai focused on the center of his instep, and more tears poured down his cheeks. Although Naruto often went around barefoot, the skin there was very smooth and soft; to make matters worse, his soles were soaked in sweat, thus allowing Sai's figures to slide more easily.

The blond boy could barely see through the tears. If so much moisture had not left his body, he would have wet himself. The stimulus in his bladder was strong, but the “excitement” somehow kept it in check. Still, his groin was not what he was concerned about.
His laughter soared up a whole octave when Sai turned his torturous attention to the upper part of his soles. He went from the instep to the base of the toes and down again like a malicious tickly spider.

“AHAHAHAAHAHAHHAHAHAHA AHHAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHAHAHAHAHA AHAHAHAHAHAHAHAHHAAH AHAHAHAHAHAHAHAHAHAHAHAHHAHAHAHAHAH”

Just when the jinchuuriki thought it couldn't get any worse, Sai hit his toes. But that feeling... it wasn't just his fingers: his skilled tongue had joined the fray. Now, the dull nails running up and down and the meaty organ darting in-between his toes, that was far worse than the armpit-balls-belly torture.
In two minutes, Naruto was but a squirming, shrilling, laughing, aching, crying rag doll. It was utterly unbearable. His life tickled out of him, he fainted.

“Oh, he's hit the bottom. I must admit he's got some crazy stamina”
When he wakes up Sai mused, looking at Naruto's dick I could make him even more sensitive. I bet his reaction would be hilarious.

Unfortunately, he didn't have the time to put his plan into practice. The sun set all too soon, and Yamato came to check the boys. The captain and Sakura were left flabbergasted when, upon entering the room, he found Sai sitting in a corner and Naruto sleeping, fully dressed again.
“He SLEPT? To think that even this method would be a waste of time...” the Wood-user complained.
“It wasn't useless, Yamato-taichou” replied the Root member, smirking, but without adding anything else.
“Whatever. Let's go back.”
While the jonin released Naruto, Sakura caught a glimpse at his feet.
Naruto sure has beautiful feet. Damn, if only I could tickle him...

A similar thought was crossing Sai's mind.
This time you were lucky. When we get back to the Leaf, I will show you everything I got.