In 1981, Arthur Fleck works as a clown-for-hire and lives with his mother, Penny, in Gotham City. The city is collapsing under unemployment, crime, and financial ruin, leaving segments of the population disenfranchised and impoverished. Arthur suffers from a neurological disorder that causes him to laugh at inopportune times, and regularly visits a social services worker to obtain medication. After a group of street kids beat him in an alley, Arthur's co-worker, Randall, loans him a gun for protection. Arthur also meets Sophie, a single mother who lives in a neighboring apartment, and invites her to his stand-up comedy routine.
While entertaining at a children's hospital, Arthur's gun falls out of his pocket. Arthur is fired for this infraction; Randall lies and says that Arthur bought the gun himself. During the subway ride home, Arthur is beaten by three drunken Wayne Enterprises businessmen; he shoots two in self-defense and executes the remaining man. The murders unintentionally start a protest movement against Gotham's rich, with protesters donning clown masks in the unidentified killer's image. Arthur later learns that funding cuts are shutting down the social service program, leaving him without access to his medication.
Sophie attends Arthur's stand-up comedy routine, which goes poorly; he laughs uncontrollably and has difficulty delivering his jokes. A popular talk show host, Murray Franklin, mocks Arthur by showing clips from the routine on his show. Arthur intercepts a letter written by Penny to local billionaire and mayoral candidate Thomas Wayne, alleging that he is Thomas's illegitimate son, and berates his mother for hiding the truth. Shortly after a visit from two GCPD detectives investigating Arthur's potential involvement in the train murders, Penny falls ill and is hospitalized. Arthur goes to Wayne Manor for answers, where he meets Thomas's son, Bruce. After a scuffle with the family's butler, Alfred Pennyworth, Arthur flees.
At a public event, Arthur confronts Thomas, who tells him that Penny is mentally unstable and not his biological mother. In denial, Arthur visits Arkham State Hospital and steals Penny's case file. He discovers that he was adopted after being abandoned as a baby and that Penny was a neglectful mother who allowed her boyfriend to physically abuse him as a child, causing serious head trauma. Distraught, Arthur goes to the hospital and kills his mother. He returns home and enters Sophie's apartment. Frightened, she tells him to leave; it is revealed that their previous encounters were Arthur's delusions.
Arthur is invited to make a guest appearance on Murray's show due to the unexpected popularity of his routine clips. As he puts on his makeup and outfit, he is visited by his former co-workers Gary and Randall. Arthur murders Randall but leaves Gary unharmed for treating Arthur well in the past. En route to the studio, he is pursued by the two detectives onto a train filled with clown protesters; one of the detectives accidentally kills a protester and incites a riot while Arthur escapes in the chaos.
Before the show goes live, Arthur requests that Murray introduce him as Joker, a reference to Murray's previous mockery. Arthur comes out to a warm reception, but begins telling morbid jokes, admits that he killed the men on the train and rants about how society abandoned and mocked him. Arthur then kills Murray before being arrested. Riots break out across Gotham. One of the rioters corners the Wayne family in an alley and murders Thomas and his wife Martha. A group of rioters in an ambulance crash into the police car carrying Arthur and free him; he is hailed as a hero by the crowd and dances to their cheers.
Arthur is imprisoned at Arkham State Hospital. Laughing to himself, his psychiatrist asks him to tell her the joke; he replies that she would not understand. He then runs down the hall being chased by the orderlies, leaving a trail of bloodied footprints he implies killed his doctor.
- Joaquin Phoenix as Arthur Fleck/Joker
- Robert De Niro as Murray Franklin
- Zazie Beetz as Sophie Dumond
- Frances Conroy as Penny Fleck
- Brett Cullen as Thomas Wayne
- Marc Maron as Gene Ufland
- Glenn Fleshler as Randall
- Leigh Gill as Gary
- Bryan Callen as a co-worker of Arthur
- Bill Camp as a Detective Garrity
- Shea Whigham as a Detective Burke
- Josh Pais as Hoyt Vaughn
- Dante Pereira-Olson as Bruce Wayne
- Douglas Hodge as Alfred Pennyworth
- Carrie Louise-Putrello as Martha Wayne
- Brian Tyree Henry as Carl
On July 10th of 2018, it was announced that Martin Scorsese would no longer be producing the film. On August 27th of 2018 it was announced that Alec Baldwin would be playing Thomas Wayne in the movie as a "cheesy and tanned businessman who is more in the mold of a 1980s Donald Trump." Two days later Baldwin told USA Today that he was no longer involved, claiming "scheduling conflicts." Brett Cullen was his replacement.