Return of the Joker, as the title suggests, features the return of Batman's old arch_nemesis the Joker, who has not been heard of since before Bruce Wayne's retirement. Wayne, who knows the reason for the Joker's long absence, is certain that this can't be the same man, and he and Terry McGinnis (the new Batman) set out to discover who it really is and what he's up to. Along the way, it is revealed what happened not only to the Joker, but also to several other Batman-related characters who have been notably absent from Batman Beyond, such as Batman's old sidekick Tim Drake.
The initial release of the movie occurred amid the backlash against violence in movies and video games aimed at children that followed the Columbine High School massacre; as a result, the movie was substantially re_edited shortly before release to tone down the violence. Many of the changes were controversial, particularly those made to a key scene in which one of the central characters is killed. The original version of the movie has since also been released.
External link
Batman Beyond: Return of the Joker (http://www.imdb.com/title/tt0233298/) at the Internet Movie Database
Batman NOT Beyond The Censor's Reach (http://www.fanzing.com/mag/fanzing32/feature5.shtml) _ detailed analysis of the last_minute changes (many spoilers)