"Joker: Folie à Deux" is the highly anticipated sequel to the 2019 film "Joker," which featured Joaquin Phoenix's Academy Award-winning performance as Arthur Fleck. The sequel will feature Lady Gaga as Harley Quinn, and the film's title, "Folie à Deux," refers to a shared delusion between two people, which reflects the complex relationship between Fleck and Quinn. The film is expected to explore the mental deterioration of both characters, with a musical component added through Lady Gaga's involvement. The sequel is also accompanied by a companion album, "Harlequin," which Gaga released to further explore Harley Quinn's transformation and madness.