"The birth went well," Minato began. The young father held his son, but otherwise the two Hokage sat alone in the room. "I was going to reinforce the seal." His wife had been the previous host of the Nine-Tails, just as the First Hokage's wife had been before her. The fūinjutsu that kept the demon at bay was at its weakest during childbirth, which was why Kushina had delivered Naruto at a supposedly secure location outside of the village.
"But someone broke in on us. He somehow got through the barrier, killed the ANBU guards...and snatched Naruto." Handing Naruto over to Hiruzen in exchange for pen and paper, the Yondaime sketched a rough impression of the man, a robed figure wearing an oval mask. The mask was white, had thin black horizontal flames decorating it, and had only one eye hole, for the right eye. That single visible eye had an iris with three black tomoe arranged around the pupil.
"I had to go for Naruto, and get him somewhere safe. The masked man took advantage of the moments I was gone to release the Kyūbi, place it under his control somehow, and when I returned to take Kushina away, he set the fox loose on Konoha."
"Many shinobi report that you were present in Konoha soon after the Kyūbi's appearance, and that you used a space-time barrier to warp the fox's Tailed Beast Ball away," Hiruzen put in. "The location you teleported the attack to is little more than a charred crater several dozen metres wide, by the way." Naruto started to cry.
"We won't be holding any more backwoods exercises there then." Minato took Naruto back, which turned out to be just what the baby wanted. "I did not act further at that point because the masked man reappeared, and tried to attack me; I escaped using the Hiraishin before he could do it. He followed me, and we fought."
"I injured him badly, I think, but I feel that he merely chose to retreat."
"After that, you returned to Konoha, teleported the Kyūbi away to Kushina and Naruto, and split and sealed it with her help, as I witnessed," the Third finished. "I thought you were going to use the Shiki Fūjin."
"That would have been safer, yes," Minato admitted. "I'm still not sure what effects sealing the Kyūbi into me will have. But Kushina insisted I find some way, any way to resolve matters such that I would not have to die, for Naruto's sake. Her final wish... So, what happened to Konoha while I wasn't there?"