The One Who Knows the Truth (真実を知る者, Shinjitsu o Shiru Mono) is chapter 397 of the Naruto manga.
On seeing Tobi's Sharingan, Sasuke's left eye takes the form of Itachi's Mangekyō Sharingan and casts Amaterasu. Tobi, caught in the flames, loses hold of his mask and falls into the shadows. Sasuke wonders what just happened. A few moments later, Tobi returns, and explains that Itachi implanted his Amaterasu into Sasuke right before he died. Sasuke remembers the moment when Itachi tapped his forehead, but asks why he would do this. Tobi tells Sasuke that it was done to protect him. Sasuke becomes angered on hearing this, but Tobi tells him that there are many things he does not know about Itachi. Tobi, revealing himself as Madara Uchiha, offers to tell him about Itachi's life. Sasuke refuses to listen, but Tobi insists, claiming it's his duty to learn the truth.