The One Who Knows the Truth (真実を知る者, Shinjitsu o Shiru Mono) is chapter 397 of the Naruto manga.
Seeing Tobi's Sharingan, Sasuke's left eye turns into Itachi's Mangekyō Sharingan and casts Amaterasu upon him. Tobi retreats into the shadows. Sasuke wonders what he did, which Tobi, upon returning, explains to have been Itachi's doing, a defence meant to kill Tobi in order to protect Sasuke. Although Sasuke remembers Itachi doing something when he tapped his forehead, Sasuke cannot understand why Itachi would want to protect him. Repeating that there are many things Sasuke does not know about Itachi, Tobi, revealing himself as Madara Uchiha, offers to tell him the truth.