Arthur Fleck (Joaquin Phoenix) is institutionalized at Arkham Asylum awaiting trial for his crimes as Joker, where all the inmates are treated brutally by the guards. While struggling with his dual identity, Arthur is led past a group of inmates taking a singing class, and is mesmerized by Lee (Lady Gaga), a female inmate who seems to have eyes for him.

The guard (Brendan Gleeson) who loves making Arthur do tricks for cigarettes, gets him a spot in the singing class, where Arthur and Lee get to know each other. Meanwhile, prior to his trial, Arthur's lawyer (Catherine Keener) tries to get Arthur sent to a mental institution where he'll get help for his mental illness brought on by childhood abuse and neglect, instead of Arkham, where he has no chance of rehabilitation.