The Secrets of the Mangekyō Sharingan

is episode 131 of the original Naruto anime.

Synopsis
After Sasuke fire's the Great Fireball Technique, he calls Naruto to show himself as he could tell that he had managed to evade it. Naruto steps out, and almost as soon as he appears, Sasuke attacks him hand-to-hand and chases Naruto back. In mid-air, Naruto manages to dodge his first few attacks before being kicked back. Sasuke then lands a devastating blow on Naruto's stomach that sends him flying and spinnig through the air and into the water again. Sasuke returns to the memory of his childhood as it begins to draw 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 traumatized 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, Naruto questions his teammate if their expieriences meant anything to him at all. He sais it was not, because it had made the two to become best-friends, being why Naruto had to die by his hands. Confused, Naruto resolves to take his best-friend down. With the flashbacks done and the tension raised, Sasuke and Naruto resume their battle.