Plot

In 1981, party clown and aspiring stand-up comedian Arthur Fleck (Joaquin Phoenix) a mentally ill, impoverished man disregarded by society, lives with his mother, Penny (Frances Conroy), in Gotham City. Gotham is rife with crime and unemployment, leaving segments of the population disenfranchised and impoverished. Even the nicest sections of the city are now looking like slums due to the breakdown of social services. Penny has been writing to famous industrialist Thomas Wayne as she used to work at the Wayne manor as a maid 30 years ago. Penny expects an answer and an offer for help from Thomas, but the letter from Thomas never arrives.

Arthur suffers from a neurological disorder that causes him to have random, uncontrollable laughing fits, requiring medication he depends on social services to obtain. Arthur is in a terminal state of depression and none of the medications are able to lift his spirits. Arthur was employed in a gig in front of a store, where he had to dress as a clown and wave a sign. Suddenly, a gang surrounds him and runs away with the sign. Arthur chases them into an alley, where the gang corners Arthur and attacks him. The sign is broken as well. Hoyt (Josh Pais) is Arthur's employer who says that the store owner is asking for his sign back and if Arthur does not return it, Hoyt will deduct it out of his paycheck. Arthur's co-worker, Randall (Glenn Fleshler), gives him a gun.

Arthur invites his neighbor, single mother Sophie (Zazie Beetz), to his stand-up comedy show, and they begin dating. Arthur had met Sophie in the lift of his apartment building and had followed her to work the next day, which she noticed. Sophie came by the same evening to Arthur's apartment and Arthur asks her out on a date.

Arthur is a big fan of the late-night talk show host Murray Franklin. Arthur attends a live screening of the show and yells out "I Love You" to Murray. As a result, Murray notices Arthur, and makes him come on stage. Arthur talks about how he takes good care of his mother and that he was put on Earth to bring joy to people's lives. Murray says that Arthur is like a son he never had, which makes Arthur's day.

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. The executives were harassing a woman travelling alone when Arthur had one of his fits of uncontrolled laughter. The executives corner Arthur and start assaulting him. Arthur shoots two in self-defense and executes the third. Arthur rushes back home and then goes over to Sophie's place to have sex with her.

The murders are condemned by billionaire mayoral candidate Thomas Wayne (Brett Cullen), who labels those envious of more successful people as "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 medication. Arthur believes that people are really starting to notice his work and feels more confident and upbeat about himself.

Arthur's comedy show goes poorly. He laughs uncontrollably and has difficulty delivering his jokes. Sophie is the only glimmer of light in Arthus's life. Arthur's idol, popular late-night talk show Murray Franklin (Robert De Niro) mocks Arthur by showing clips from the routine on his show and calls him a "joker", much to Arthur's distress. Arthur intercepts a letter written by Penny to Thomas, alleging that he is Thomas's illegitimate son, and berates his mother for hiding the truth.

At Wayne Manor, Arthur talks to Thomas's young son, Bruce (Dante Pereira-Olson), but flees after a scuffle with butler Alfred Pennyworth (Douglas Hodge). 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 reads Penny's file, which states she was a delusional narcissist who adopted Arthur while working as a housekeeper for the Waynes. She developed an imaginary intimate relationship with Thomas, which included Arthur being their biological son. At the same time, Penny raised Arthur with a boyfriend who physically and sexually abused them, leading to his neurological disorder. A distraught Arthur enters Sophie's apartment unannounced. Frightened, she kindly asks him to leave, revealing their relationship to be a figment of Arthur's imagination. The following day, Arthur smothers Penny with a pillow 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 but leaves Gary unharmed for treating him well in the past. On route to the studio, Arthur is pursued by the two detectives onto a train filled with 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 tells morbid jokes, admits he killed the men on the train, and rants about how society abandons the disenfranchised. Arthur shoots Murray and is arrested as riots break out across Gotham. Looters in an ambulance crash into the police car carrying Arthur and free him. Meanwhile, one rioter corners the Wayne family in an alley and murders Thomas and his wife Martha in front of Bruce. Arthur stands atop the police car, dances to the cheers of the crowd, and smears the blood on his face into a smile.

Sometime later at Arkham, Arthur laughs to himself about a joke during a session with his new therapist. He declines telling it to her, affirming she would not understand it. He leaves behind a trail of bloodied shoe-prints as an orderly chases him down.