"Roar, Chidori! Brother vs. Brother!" (唸れ千鳥 吠えろサスケ!, Unare Chidori, Hoero Sasuke!) is episode 84 of the original Naruto anime.
As Naruto opens the door and finds himself face to face with Itachi, he realises that he looks just like Sasuke and has the Sharingan eyes as well. Meanwhile, Sasuke is running through the streets, desperate to find Itachi. While running, he reminisces on his past. When he was younger, Sasuke had always looked up to his brother. One morning, he had asked Itachi to help him with his shuriken training, only for Itachi to brush him off. Sasuke then asked his father how come Itachi never had time for him. His father told Sasuke that Itachi was very closed off and distant, and that not even he knew his own son very well. Sasuke's mother then gave Sasuke his lunch, and Sasuke headed off to the Academy. On the way there, he came across a relative of his, who mentioned that by his age, Itachi had already awakened his Sharingan and had become a genin. Sasuke was initially put down, but his uncle Teyaki assured him that in time, he would be a great shinobi as well, for he was an Uchiha like his brother. After school, Sasuke rushed home, fearing he was late as the sun had already set. Sasuke then noticed the bodies of his relatives littered across the streets, with blood, shuriken and kunai on the walls and doors. Fearful, Sasuke rushed home, only to find his parents dead. Realising it was Itachi who was responsible, Sasuke demanded to know why he had slaughtered their entire clan. Itachi claimed that he had murdered everyone to test his strength. Sasuke ran outside and begged to live, to which Itachi stated that he was not even worth killing. He then told Sasuke to live the rest of his life with hatred and vengeance towards him.
Back in the present, as Kisame and Itachi contemplate how best to carry Naruto, Sasuke arrives. Kisame is shocked at seeing Sasuke, not only because he looks similar to Itachi, but because he too possesses the Sharingan. Itachi tells him that Sasuke is his younger brother, which Kisame finds funny as he had believed that Itachi had murdered every single Uchiha. Sasuke tells Itachi that he has been living this entire time with hatred in his heart, his only purpose in life to kill Itachi. Sasuke then rushes at Itachi with his Chidori. Uninterested in their quarrel, Itachi easily stops the attack. To try and help Sasuke, Naruto uses the Nine-Tailed Demon Fox's chakra in an attempt to summon a toad, though Kisame uses his blade to absorb and devour the chakra. As Kisame prepares to attack Naruto, a toad arrives to block the would-be attack, followed by Jiraiya.