If I remember right Robin teamed up with Slade to find Raven while the other Titans distracted Tritan. Then Raven got pissed and sent Tritan back to his own dimension reversing every hit g that happened. In the next season they fought an alliance of villains ( I can't remember the name.) led by the Brain. Originally there were just 5 of them, but they recruited every villain from the show up to that point excluding Slade and Tritan. The Titans responded by gathering teen heroes from across the world. In the end the evil league captured all of the Titans one by one except for Beastboy and a handful recruits. That group attempted to rescue Robin and the other heroes and a final battle ensued. The final episode was just a weird deal with Beast Boy remembering Terra.