"A Mistake from the Past: A Face Revealed!" (火影の過ち 仮面の下の素顔, Hokage no Ayamachi Kamen ni Shita no Sugao) is episode 72 of the original Naruto anime.
Sasuke finally manages to catch up to Gaara, but Temari stalls him so that Kankurō and Gaara can escape. He easily deals with her and resumes his chase. At the Hokage battle, Orochimaru shows the Third Hokage the fruits of his quest to gain immortality. Peeling away his face, Orochimaru reveals the face of a girl whose body he has taken; Orochimaru's key to living forever is to take the bodies of others to extend his lifespan, and he reveals his intentions of stealing the body of Sasuke Uchiha to use as his new body. The Third Hokage regrets not killing Orochimaru years before when he had the chance. Finally ready to take down Orochimaru, the Third Hokage musters up his chakra and summons shadow clones and uses the remaining chakra to perform the Fourth Hokage's forbidden jutsu - the Dead Demon Consuming Seal. Despite the Second Hokage using a genjutsu that causes complete darkness, the Third Hokage manages to hold on. Orochimaru claims that he will destroy Konoha just as he will destroy the Third Hokage. The Third states that the Hokage that succeed him will continue to be the pillar of Konoha and protect the village; he then remembers the past and current inhabitants of Konoha that he fights to protect. With that, the Third rushes forwards to begin his attack.