The Secrets of the Mangekyō Sharingan (開眼 万華鏡写輪眼の秘密, Kaigan Mangekyō Sharingan no Himitsu) is episode 131 of the original Naruto anime.
As Sasuke's remembrance of his childhood draws to a close, Sasuke returned home from the Ninja Academy one day to find all the members of the Uchiha clan murdered. Upon finding his parents' bodies, Sasuke discovered Itachi to be responsible for the massacre. Using his Mangekyō Sharingan, Itachi showed Sasuke their parents' deaths, causing Sasuke to run with fright. Itachi stopped him, telling him that he wasn't worth killing. As Itachi left, he informed Sasuke that if he wanted to take revenge for the clan, he would need to get stronger. To help Sasuke gain the strongest ability of the Uchiha, Itachi tells Sasuke the requirements to obtain his own Mangekyō Sharingan: He must kill his closest friend. With that, Itachi left, leaving Sasuke to build a hate for his brother. Resolving to get stronger, Sasuke decided he would do whatever was necessary to kill Itachi. Returning to the present day, Sasuke and Naruto resume their battle.