In an animated short, the Joker is impersonated by his shadow during a TV performance, leaving him half-undressed and vulnerable before three policemen arrive and beat him.

In 1983, Arthur Fleck is at Arkham State Hospital awaiting trial for crimes committed two years earlier. His lawyer, Maryanne Stewart, plans to argue he has dissociative identity disorder, attributing his crimes to his Joker persona. During music therapy, Arthur meets Harleen "Lee" Quinzel, who shares her traumatic past and admiration for the Joker.

At a film screening, Lee starts a fire, and both are caught trying to escape. While Arthur is placed in solitary, Lee visits him, claiming she is being released to avoid his influence but promising to attend his trial. They share a passionate moment in the cell, and Arthur later sings to her during a TV interview, deepening her affection.

On trial day, Assistant District Attorney Harvey Dent calls witnesses who undermine Arthur’s insanity claims. During a break, Maryanne reveals that Lee is actually a psychiatry student with a fabricated past. Confronted, Lee admits her lies were attempts to connect with Arthur but also reveals she’s pregnant and has moved into his old apartment.

At trial, Arthur dismisses his lawyer and represents himself. Witness testimonies deeply affect him, leading to a lack of defense. After being brutally beaten by guards, he hears his friend Ricky being strangled for confronting them. This brings back memories of his first kills as the Joker.

In his closing argument, a devastated Arthur renounces his persona, Lee storms out, and he’s found guilty. A car bomb detonates outside the courthouse, allowing two followers to help him escape. Arthur finds Lee outside his old apartment, but she rejects him, revealing her pregnancy was a lie. The police recapture him, and a young patient fatally stabs him, carving a smile into his own face as Arthur bleeds out and dies.