'Joker: Folie a Deux' stars Lady Gaga as Harley Quinn, alongside Joaquin Phoenix, who is reprising his Oscar-winning role from the first installment. The story follows Arthur Fleck (Phoenix) as he awaits trial for the crimes he committed as the Joker. While struggling with his dual identity, Arthur not only comes across true love, but discovers the music that has always lived within him. 'Joker: Folie �� Deux' is in theaters on Oct. 4.