"The Horrible Experiment…!!" (恐るべき実験…!!, Osorubeki Jikken…!!) is chapter 121 of the original Naruto manga.
As Sarutobi is about to perform his last technique, Orochimaru removes his face, revealing that of a young woman underneath. Realising that Orochimaru has learned how to switch to others' bodies as a form of immortality, Sarutobi thinks back on how Orochimaru came to be expelled from Konoha; how he had caught Orochimaru experimenting on his fellow Konoha ninja in order to achieve immortality. With Orochimaru finally having proven successful, and already knowing of his interest in Sasuke, Sarutobi states that Orochimaru's true intentions for this invasion of Konoha are to gain the Sharingan.