"Harley Quinn's Revenge", a story based campaign expansion, was released on May 29, 2012, for the PS3 and the Xbox 360, with a PC version coming out a week later. The campaign features a new story, new areas, new enemies, and both Batman and Robin as playable characters. The story takes place two weeks after the events of Arkham City. The mega prison has since been evacuated, but Quinn returns and sets up in Joker's former base. The game follows Robin's search for Batman, who has gone missing while hunting Quinn;Batman has been acting differently following the end of the main game story, concerning his allies. On the same day, it was announced that all of the released DLC, including "Harley Quinn's Revenge", would be made available as part of the Arkham City Game of the Year edition.
Source: Wikipedia, "http://en.wikipedia.org/wiki/Batman:_Arkham_City," available under the CC-BY-SA License.