October 10th. The night of the Kyuubi attack. What a mess of a night. A certain sorcerer once referred to this as a nexus event. A series of events that, try as one might to prevent them, were always destined to happen. Regardless of the world. Regardless of the universe. Some events will always happen in one form or another, however it is the slight differences that distinguish one version of such events to the next.
In the prime continuity of this branch of the omnipresent force, known to magus as the Root of Existence. The Fourth Hokage and his wife gave their lives to protect their only child, after being forced to give the ultimate sacrifice, and imprison the Kyuubi within their son, Naruto Uzumaki. Naruto would then grow up as a pariah in his own home village, until eventually proving that he was not the beast, and earning the love and respect of his people. Then later achieving his dream of becoming Hokage, the leader and protector of his home.
This, however, is not the prime continuity.
In this universe, this insignificant leaf, jutting out on the branch designated for all of the Multiverse that centers around the many lives of Naruto Uzumaki. Naruto was not made a Jinchuuriki, and his parents were not killed. After all, they had another option.
During the attack, with the limited time they had available, the Fourth Hokage Minato Namikaze and his wife Kushina Uzumaki decided to minimize any unnecessary risks. For the last two generations, the Kyuubi had been imprisoned using sealing techniques placed upon the bodies of Uzumaki women, and it was known that they could handle the strain. So they decided to continue the line and sealed the beast into the body of their newborn daughter Yui Namikaze. However, Minato was a genius with seals and had long since devised a plan to kill the immortal beast.
For the Tailed Beasts or Biju as they were also called, of which the Kyuubi was the 9th and strongest, death was only temporary. At the time of their demise, their Demonic Chakra, the acidic manifestation of their life force, would scatter throughout the world and over the course of a year, slowly gather back together and revive the once fallen beast.
With this fact known, Minato's plan was both simple and brilliant. Separate the beast's soul from its Chakra, and before it can dissipate, seal only the chakra away and leave the soul to fade away with no hope of ever reuniting with its Chakra. The plan was perfect, it made pure logical sense and should have been a perfect success, but that was its downfall.
For you see, the Tailed Beasts were inherently illogical. They were not created by any normal means, they were created when the Sage of the Six Paths split the Primordial God of Chakra, the Juubi, into nine pieces. They were illogical, because they were not creatures, they were pieces of a god, and as such followed laws differing to mortals. Their deathless existence should have been Minato's first clue that something would go wrong.
With the sealing of its Chakra, the Kyuubi immediately understood their situation. They realized that while they would not die entirely, they would also never reform as the seal would slowly filter his chakra into the hosts Chakra Network. Making his power theirs, just as all seals before. It was as close to death as a Biju could get. So the Biju attempted his final Gambit, willingly entering the confines of a child's body.
As nothing more than a spirit, the Kyuubi attempted to enter the body of Yui and was rebuffed by the seal. The seal's safety matrix would prevent the soul from reuniting with the Chakra at all costs. So, in the midst of panic that the beast had never known before, an idea sprouted.
"If I cannot regain my chakra, then I will simply need to prevent myself from fading until I can create enough to sustain myself."
And with that final thought, the Kyuubi entered the body of a child for the third time, only this time, it was without being sealed. And that, dear reader, was the shatter point that caused this story to diverge from the script.