Joker’s version of the Batman origin story confirms that the events of the film aren’t real, but rather took place in Arthur Fleck’s mind. Although released alongside entries in the DCEU, Joker was billed as a standalone movie - though its box-office and critical success might cause DC to push for sequels. Directed by Todd Phillips and starring Joaquin Phoenix, Joker tells the tale of Arthur Fleck, a struggling comedian with intense mental health problems, whose world is falling down around him. Over the course of the film, he becomes the titular Joker and causes a revolution within Gotham City, leading to the death of Bruce Wayne’s parents - an event that, famously, acts as Batman’s origin story.
Since Joker’s release in 2019, fans have theorized that the events of the film are in actual fact largely a fantasy concocted in Arthur’s troubled mind, with much of what happens reflecting more what Fleck wants from life than legitimate reality. While the film certainly hints at this, with Arthur’s relationship to both Sophie Dumond (Zazie Beetz) and Murray Franklin (Robert De Niro) revealed to be false on multiple occasions, other events are open-ended enough to consider other theories. That said, Arthur’s memories of the Wayne parents being murdered at the end of the film help to confirm that the events of Joker are likely not real, in-universe.
During the film’s climax, riots break out all over Gotham after Arthur murders Murray Franklin on live TV. While Arthur is arrested and bundled into a police car (which later crashes, allowing Arthur to be pulled free), rioters shoot Thomas and Martha Wayne in an alleyway but spare the couple’s young son - Bruce Wayne. In Joker’s final scene, which takes place in Arkham State Hospital, Arthur remembers the Waynes being murdered - something that he couldn’t have witnessed from his position inside and then atop the police car. For this reason, the events of the film almost certainly take place inside Arthur’s head.
Were this to hold true, it could be one of the most telling scenes in all of Joker, as it suggests the reason he thinks the riots (real or fake) were justified is because they killed the man he'd made into his personal "villain" - Thomas Wayne. Arthur's fantasies appear to verge on wish fulfillment - as shown best with his imagined relationship with Sophie - and so this can be seen as the point of no return for him, as Fleck goes from wanting a familial relationship with Thomas Wayne, to visualizing and seemingly delighting in his brutal murder. Of course, as this is a retrospective narrative, it can also be interpreted that Arthur is simply recalling all of the "important" events that occurred this one fateful night - but the manner in which he does so still calls into question how accurate his interpretation of events can be.
This all said, Joker is a film that is best left up to interpretations, as movies like American Psycho have showcased that an unreliable (and unstable) narrator allows a level of mystery into a plot that leaves audiences questioning what exactly happened for years to come. And with the high level of critical praise placed on Joker, it seems having anything resembling a solid answer in terms of Fleck's narrative accuracy would only dampen the overall film, as much of the excitement lies in its uncertainty.