Quick recap
During the Third Ninja War, Kakashi, still a kid, has just made jōnin and lives by the rule that the mission comes first, shaped by the fate of his father, the White Fang. Team Minato is sent to destroy Kannabi Bridge. Minato splits off, and Rin is captured by the Stone. Kakashi wants to stick to the mission; Obito defies him and goes to save her, and Kakashi ends up following. In the fight Obito awakens his Sharingan, but he's pinned under a boulder saving Kakashi. Before he's buried, he gives his eye to his friend. Minato arrives in time to get Kakashi and Rin out, and Kakashi carries Obito's Sharingan from then on.
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