Hokuto no Ken Episode 35
Yuu has become sick, so his mother and father take him to the village of miracles, a place they have been told has a doctor who can cure anyone. Lin goes with them, as she thinks this doctor may by Toki, who Rei mentioned previously. As they arrive and head in, Kenshiro and Habu also appear at Toki's base. Inside, Toki agrees to help save Yuu, but only if they will help him. He hits pressure points on Yuu's body and says he will be fine shortly, but he then demands that his father gives his life up.
Kenshiro then enters (Habu is stabbed and killed accidentally by Toki) and sees Yuu. He has to hit more pressure points to save his life, as what Toki had actually done was hit points that would eventually kill him. Ken and Toki clash, but Toki easily blocks or avoids all of his opponent's attacks. He explains to Kenshiro why he became evil -he tried to save as many people as he could in the village of miracles, only for a gang to show up and murder everyone. Toki went crazy, completely slaughtering the entire gang. When he came back to his senses, he realised his goal of of using Hokuto Shinken as a medical art was pointless, and he should use it as it was originally intended -an art for killing.
Yuu's father tries to attack Toki, only to have a pressure point hit which causes his heart to explode. Kenshiro attacks Toki again, but is devastated when he sees Toki's back -it has a large scar on it, meaning he is the real Toki after all (Kenshiro witnessed Toki getting this scar a long time a go). Toki says that it is absurd that Kenshiro was made into the successor of Hokuto Shinken, but Ken is thinking about his destiny: he has already killed one of his brothers, now it looks like he will have to kill another. The two men stare at each other and prepare to enter a final confrontation.
In the manga...
-Yuu and his mother and father make their first appearance in the manga when they come across Toki's place -Yuu is already sick. In the anime, they've been around since the previous episode, with new scenes involving them finding Lin and being part of a traveling merchant group.
-Lin is not present with Yuu's family. She's still back in Mamiya's village in the manga.
-Yuu dies in the manga. When Kenshiro enters the room, he can immediately tell that Toki has hit pressure points that have killed him. In the anime they changed it so Ken saves him in time.