Remaining Ambition: Orochimaru's Shadow

is episode 146 of the original Naruto anime.

Synopsis
Naruto, Iruka, and Pakkun meet up with Mizuki's fiancée Tsubaki, and she has come to offer help to the three, feeling responsible for not having stopped him earlier. Meanwhile, Shikamaru, Ino, and Choji are having trouble holding the two brothers off, and they know that they can't defeat them, so they are stalling for time. Meanwhile, Tsunade discovers a startling secret about Mizuki and Orochimaru, and leaves to intervene. Naruto, Iruka, Pakkun, and Tsubaki finally reach Mizuki, but it's too late, he's already completed the formula he was making. Now that he's drank it, he turns into a new monstrous form. And, Naruto has recognized Mizuki's transformation as one nearly identical to that of Orochimaru's cursed seal.