aka Batman of the Future: Return of the Joker (2000)
The only feature-length outing for Batman Beyond is a great one. It came out between Seasons 02 and 03 of the parent series, so was able to reference Terry McGinnis' life up to that point, including splicing and The Jokerz street gang. [1]
It's no spoiler to say that The Joker (Mark Hamill) appears to have returned, which impacts the lives of both Bruce Wayne (Kevin Conroy) and Terry (Will Friedle). The Clown Prince of Crime's appearance vexes Bruce greatly and is the mystery that drives the narrative in the early part.