Paradox
Or,
Why Time Travel Should Never Ever Be Attempted Under Any Circumstances
Konoha was burning. Well, not precisely. Actually, it was all smashed up and currently suffering from an invasion of Sound ninja, but burning sounds cooler so we'll just stick with that.
As it so happens, the combined might of Jiraiya and Tsunade together could not subdue Orochimaru (even though they're supposed to be equal in strength). Let's call it a plot hole and leave it at that.
The civilians were mostly dead, but we don't really care about them. They aren't important. Well, except Teuchi and Ayame, but they were long gone.
Most of the Konoha Twelve was dead too— everyone except Sasuke and Naruto had perished in said invasion, and Sasuke was no longer in existence upon this earth (it's a fancy way of saying he'd dead too).
Has anyone ever told Orochimaru that he shouldn't go around stealing little boy's bodies?
Just in case you're wondering, Kakashi was dead. Gai, Asuma, and Kurenai were alive, but nobody ever features them in any fanfictions. Gai would say that is not very youthful, but we're ignoring him, remember?
That left Naruto as the only useful Leaf ninja alive the duty of eradicating Orochimaru.
It was a battle long and hard, involving rasenshurikens a-flyin' and long necks a-twistin' and a strangely large number of high-powered fire jutsus (considering that neither participant much liked those techniques).
In the end, though, since Orochimaru was immortal (but not really since later on in this story Naruto has to kill him), the Snake Sannin prevailed. And, seeing his foe bleeding out his life on the ground, Orochimaru raised his sword and…
… set it to the side as he pulled a piece of flower-patterned stationary out of one sleeve. "BWAHAHAHA!" he cackled. "I, the great and mighty Orochimaru, have prevailed! You, my dear foe, will die very soon. I love Jiraiya's Icha Icha series and kidnapping young children. Most people call me a—" Finally he seemed to realize what he was reciting. A Mangekyo Sharingan appeared in his eyes. "Amateseru." the Sannin growled, as the paper burst into black fire. "When I find whoever messed with my victory speech…"
Meanwhile, Naruto was not paying attention to the man at all. Instead, he found himself inside the sewer that is his mind.
The Fox began speaking rapidly as soon as he saw his jinchuriki. NARUTO, THERE IS A TECHNIQUE KNOWN TO ONLY THE BIJU THAT WILL TURN BACK TIME. WE COULD USE THIS TO GO BACK AND STOP OROCHIMARU BEFORE HE TAKES THAT UCHICHA YOU WERE SO FOND OF, THEREBY AVERTING THE FALL OF KONOHA…
Naruto grinned. "What do we have to do, Kurama?"
YOU? NOTHING. JUST SIT BACK AND DON'T DIE WHILE I STEAL MOST (ALL) OF YOUR CHAKRA FOR A POTENTIALLY DANGEROUS JUTSU THAT WILL PROBABLY KILL BOTH OF US.
"All right!" our ever-cheerful protagonist exclaimed. "Let's do this!"
A minute later, Orochimaru had finished reciting his victory speech (the real one this time). But it didn't matter, because just at that moment and every moment before and every moment that would never come after, the universe ceased to exist and just before he had never existed, Kurama remembered something.
"Now," Hadoromo said sternly, "I am going to teach you a technique that I came up with, my children. However, you must NEVER EVER USE IT. I am teaching it to you ONLY because it includes some very interesting theory. This jutsu is for time travel. The first thing you must know about the universe is that it cannot stand paradoxes— hence why this jutsu must never be utilized. Time travel creates paradoxes, and when confronted with a paradox this world will cease to exist. Please keep that in mind. Now, on to the lesson…"
OOPS.