Kakashi Chronicles ~ Boys' Life on the Battlefield ~ Part 2

Kannabi Bridge, the mission that made Kakashi. Pinned in a collapsing cave behind enemy lines, Obito Uchiha awakens his Sharingan too late to matter and gives his left eye away as a field promotion gift — Rin transplanting it into Kakashi's ruined socket while the rocks come down. Obito's last words are a joke and a request; the boulder takes the rest. The episode closes the flashback that explains everything about the man under the mask: the chronic lateness, the rule about comrades, the grave he visits before every mission.