In 1981, party clown and aspiring stand-up comedian Arthur Fleck lives with his mother, Penny, in Gotham City. Gotham's class society is rife with crime and unemployment, leaving segments of the population disenfranchised and impoverished. Arthur suffers from a medical disorder that causes him to laugh at inappropriate times, and depends on social services for medication. After a gang of delinquents attacks Arthur in an alley, his co-worker, Randall, gives him a gun for protection. Arthur meets his neighbor, single mother Sophie Dumond, and invites her to his upcoming stand-up comedy show at a nightclub.
While entertaining at a children's hospital, Arthur's gun falls out of his pocket. Randall lies that Arthur bought the gun himself and Arthur is fired. On the subway, still in his clown makeup, Arthur is beaten by three drunken Wayne Enterprises businessmen who were harassing a female passenger; he shoots two in self-defense and executes the third. The murders are condemned by billionaire mayoral candidate Thomas Wayne, who labels those envious of more successful people "clowns". Demonstrations against Gotham's rich begin, with protesters donning clown masks in Arthur's image. Funding cuts shutter the social service program, leaving Arthur without his medication.
Sophie attends Arthur's stand up routine, which goes poorly; he laughs uncontrollably and has difficulty delivering his jokes. Popular talk show host Murray Franklin mocks Arthur by showing clips from the comedy routine on his show. Arthur intercepts a letter written by Penny to Thomas Wayne, alleging that he is Thomas's illegitimate son, and berates his mother for hiding the truth. Arthur goes to Wayne Manor, where he meets Thomas's young son Bruce, but flees after a scuffle with Alfred Pennyworth, the family butler. Following a visit from two Gotham City Police Department detectives investigating Arthur's involvement in the train murders, Penny suffers a stroke and is hospitalized.
At a public event, Arthur confronts Thomas, who tells him that Penny is delusional and not his biological mother. In denial, Arthur visits Arkham State Hospital and steals Penny's case file; the file says Penny adopted Arthur as a baby and allowed her abusive boyfriend to harm them both. However, Penny alleged that Thomas used his influence to fabricate the adoption and commit her to the asylum to hide their affair. Distraught, Arthur returns home and enters Sophie's apartment unannounced. Frightened, Sophie tells him to leave; their previous encounters were Arthur's delusions. The following day, Arthur kills Penny at the hospital.
Arthur is invited to appear on Murray's show due to the unexpected popularity of his comedy routine's clips. As he prepares, Arthur is visited by Randall and fellow ex-colleague Gary. Arthur murders Randall for revenge, but leaves Gary unharmed for treating him well in the past. En route to the studio, Arthur is pursued by the two detectives onto a train filled with some of the clown protesters. One detective accidentally shoots a protester and incites a riot, allowing Arthur to escape.
Before the show goes live, Arthur requests that Murray introduce him as Joker, a reference to Murray's previous mockery. Arthur walks out to applause but the mood quickly changes when he tells morbid jokes, confesses to the train murders, and rants about how society abandons the downtrodden and mentally ill. Arthur kills Murray and is arrested as riots break out across Gotham. One rioter corners the Wayne family in an alley and murders Thomas and his wife Martha, sparing Bruce.[a] Rioters in an ambulance crash into the police car, freeing Arthur, who dances to the cheers of the crowd.
At Arkham, Arthur laughs to himself about a joke, but refuses to tell it to his psychiatrist. He then runs from the orderlies, leaving a trail of bloodied footprints.
