Naruto had one enormous question weighing on his mind: Madara Uchiha's Rinnegan. Awakening the Rinnegan required both Indra's and Asura's chakra, and Madara Uchiha had clearly awakened his after obtaining the First Hokage's cells. But there was a huge mystery in all of this: how had the First Hokage known?
Clearly, the First Hokage had transplanted his own cells into Madara Uchiha to help him awaken the Rinnegan. That was why Tsunade had said Madara Uchiha had awakened the Rinnegan rather than the Eternal Mangekyō Sharingan. But what about the First Hokage? Had he awakened the Rinnegan too? How had he known? Could he have been a transmigrator like Naruto? But that was impossible. If the First Hokage had been a transmigrator, he would never have left a menace like Black Zetsu alive, much less allowed an organization like the Akatsuki to emerge. So Naruto could be fairly certain the First Hokage wasn't a transmigrator. How, then, had he known all this?
Naruto thought about it for a long time but couldn't make sense of it. In the end, he could only ask, "Granny Tsunade, you said Madara Uchiha's eyes evolved into the Rinnegan. How exactly did that happen?"
Tsunade replied, "That, I don't know. I remember the First Hokage and Madara Uchiha disappearing together for a while. No one knew what they did during that time. All we knew was that when they returned, Madara Uchiha already had the Rinnegan, and they'd brought back eight tailed beasts. Aside from the One-Tail in the Land of Wind, the First Hokage and Madara Uchiha brought all the others back to Konoha. It was the Rinnegan that controlled the tailed beasts then."
Hearing this, Naruto hurriedly asked inwardly, Kurama, you heard that. What exactly happened? How did they capture you back then?