Noticing that one of her fellow inmates in Arkham, the Riddler, is depressed after having recently been taken here after yet another defeat at the hands of the Dark Knight, Harley Quinn decides to organize a little escape for a getaway to the capital of Belgium, Brussels. In the company of the Scarecrow, Killer Croc and, of course, her "pudding", the Joker, they take advantage of a big costume party in the European city to party to unwind a little and forget their worries because of the Bat. However, the next morning, Harley, Joker, and Croc wake up with the worst hangover of their lives, with no memory of what happened the day before ! The scarecrow, who had left the old woman for a "doctor's seminar" in town, finds them and begins to try to help them understand what happened and, above all, to find Nigma, who's disappeared !
