In a striking animated sequence, the Joker finds himself replaced by his own shadow during a singing number for a television show, only to be left behind on stage as three policemen arrive and brutally assault him.
Now in custody at Arkham State Hospital, Arthur Fleck is awaiting trial for crimes he committed two years earlier. His lawyer, Maryanne Stewart, is preparing to argue that Arthur suffers from dissociative identity disorder, attributing his violent actions to the dark persona of the Joker. At a music therapy session in the hospital’s B ward, Arthur encounters Harleen "Lee" Quinzel, played by Lady Gaga. Lee shares her tragic backstory, revealing her abusive childhood and how she was admitted after a devastating incident involving her parents' home. Intriguingly, she also expresses admiration for Arthur's Joker persona and his notorious deeds.
As they watch a screening of The Band Wagon, chaos ensues when Lee ignites a fire, leading to their capture while attempting to escape. This results in Arthur being placed in solitary confinement. Lee, visiting him, shares her plans to be released in order to distance herself from his influence but promises to support him during his trial, culminating in a brief intimate encounter. During an interview with television host Paddy Meyers, Arthur serenades Lee through the screen, deepening her infatuation with him.
On the trial's day, Assistant District Attorney Harvey Dent presents witnesses who invalidate Arthur's insanity claims. In a surprising twist, Maryanne unveils Lee's true identity as a psychiatry student from the Upper West Side whose father is very much alive. She had voluntarily committed to Arkham and had never burned down her childhood home. Confronted by Arthur, Lee admits her fabrications were efforts to bond with him but drops the bombshell that she is pregnant and has moved into his former apartment building, wishing to forge a life together.
The trial takes a dramatic turn as Arthur dismisses Maryanne, opting to represent himself. After hearing testimonies from Gary Puddles Leigh Gill and Sophie Dumond Zazie Beetz, Arthur is noticeably shaken and offers no defense. Instead, during his speech, he ridicules the Arkham guards and hints at the abuse he has suffered. Sent back to Arkham, he endures a horrific assault by head guard Jackie Sullivan and others, culminating in the death of his friend Ricky Jacob Lofland.
In a heart-wrenching closing statement in court, Arthur resolutely renounces his Joker identity, taking accountability for his actions. This revelation angers Lee, prompting her to leave his side. The ensuing verdict finds Arthur guilty of first-degree murder, and as the foreman announces the verdict, a car bomb detonates outside the courthouse, causing mass casualties and leaving Dent severely scarred.
In the ensuing chaos, Arthur’s followers facilitate his escape. Wandering through Gotham City, he finds Lee outside his former residence, but she turns him away for renouncing the Joker. As Arthur is recaptured by the police, he faces betrayal from a young inmate who approaches him with a joke before brutally stabbing him. As Arthur succumbs to his injuries, the assailant laughs maniacally, carving a haunting smile on his own face, adding a sinister twist to Arthur's tragic story.
