Arthur Fleck is a professional clown and aspiring stand-up comedian who lives with his mother, Penny, in crime-ridden Gotham City during the early 1980s recession. He suffers from a neurological disorder that causes him to have random, uncontrollable laughing fits, requiring medication for which he depends on social services to obtain. After Arthur is attacked by street urchins, his co-worker Randall gives him a revolver for self-defense. Arthur pursues a relationship with his neighbor, single mother Sophie, and invites her to see his routine at a comedy club.
Arthur is fired from his job when he accidentally exposes the gun while performing at a children's hospital, despite his explanation that it was given to him by Randall, who denies this. While riding home on the subway, still wearing his clown make-up, Arthur is mocked and assaulted by drunk businessmen from Wayne Investments; he fatally shoots two of them in self-defense and murders the third as he attempts to flee. The killings are condemned by their employer, billionaire mayoral candidate Thomas Wayne, but protesters begin donning clown masks in Arthur's image. Budget cuts shut down the social service program, leaving Arthur without his medication.
Sophie attends Arthur's stand-up routine, which goes poorly; he laughs uncontrollably, as his jokes fall flat. Arthur intercepts a letter from Penny to Thomas, alleging that he is Thomas's illegitimate son, and berates his mother for hiding the truth. He goes to Wayne Manor, where he meets Thomas's young son Bruce, but flees after a scuffle with family butler Alfred Pennyworth. Penny suffers a stroke and is hospitalized. Arthur's idol, popular late-night talk show host Murray Franklin, presents clips of Arthur's failed performance on his show and mockingly calls him a "joker".
Arthur confronts Thomas at a film theatre. Thomas tells him that he is not his father, and Penny is not his biological mother. In denial, Arthur visits Arkham State Hospital and reads Penny's file, which states she was a narcissist who adopted Arthur while working as a housekeeper for the Waynes in the 1950s. Penny then raised Arthur with her abusive boyfriend, who later died in jail. Penny was sent to Arkham for allowing the abuse. Distraught, Arthur enters Sophie's apartment unannounced. Frightened, Sophie asks him to leave, revealing their relationship to be a figment of Arthur's imagination. The following day, Arthur smothers Penny to death with a pillow at the hospital.
Arthur is later invited to appear on Murray's show. He subsequently plans to kill himself with his revolver during the broadcast. While crafting a clown-inspired persona, he is visited by Randall and another ex-colleague, Gary. Arthur fatally stabs Randall, but spares Gary for being kind to him in the past. Two detectives investigating Arthur's involvement in the murders of the businessmen pursue him onto a subway filled with clown protesters. Arthur incites a brawl, during which one detective shoots and kills a protester. The other protesters, in turn, beat the detectives while Arthur escapes.
At the studio, Arthur asks Murray to introduce him as "Joker", a reference to his earlier mockery. As the show goes live, Arthur forcefully kisses a guest, tells morbid jokes, confesses to the subway murders, rants about how society abandons the downtrodden and mentally ill, and berates Murray for ridiculing him. After a final joke, Arthur fatally shoots Murray in the head, on air. He is arrested, and riots erupt across the city. Rioters in an ambulance crash into the police car carrying Arthur and free him. Meanwhile, a rioter corners the Wayne family in an alley and murders Thomas and his wife, sparing Bruce. Arthur stands atop the police car, starts to dance to the cheers of the crowd, and smears blood on his face into the shape of a smile.
At Arkham, Arthur laughs to himself about a joke while with a therapist, but declines to tell it to her, claiming she would not understand it. He leaves behind a trail of bloodied shoeprints as he is chased down a corridor by an orderly.
