The central twist of "Joker" is the revelation that Arthur's romantic relationship with his neighbor, Sophie Dumond, is entirely a figment of his imagination. Throughout the film, she is shown supporting him at his comedy show and comforting him at the hospital. The reveal comes late in the film when Arthur lets himself into her apartment, and the terrified real Sophie barely recognizes him, asking him to leave. The film then flashes back through their 'shared' moments, showing Arthur alone in each instance. This twist retroactively reframes the entire narrative, confirming Arthur as a profoundly unreliable narrator and establishing the depth of his delusion and isolation.
Another major revelation concerns Arthur's parentage. His mother, Penny, has led him to believe he is the illegitimate son of Thomas Wayne. This is seemingly debunked when Arthur confronts Wayne and later reads his mother's file from Arkham Asylum, which states she was delusional and that Arthur was adopted and horrifically abused as a child by one of her boyfriends, causing a significant head injury that may be the source of his laughing condition. However, the film leaves a small photograph of a young Penny with a loving inscription from 'T.W.' on the back, leaving a sliver of ambiguity. The climax sees Arthur, as Joker, murder his idol Murray Franklin on live television, an act that ignites city-wide riots. In the ensuing chaos, a rioter wearing a clown mask murders Thomas and Martha Wayne in an alley, an ironic twist where Arthur indirectly creates his own nemesis, Batman. The film's final scene in Arkham suggests the entire story might have been a joke or a delusion he concocted, as he tells his therapist, 'You wouldn't get it,' before walking away, leaving bloody footprints.