Unlike the Justice League series of films that spanned from Man of Steel to Aquaman, Joker is not part of a larger universe. This film is separate from the movies starring Ben Affleck, and also the upcoming The Batman, starring Robert Pattinson. But does that mean Joker is a completely Bat-free film? Not quite. The ending, in particular, has fans asking, is Joker a Batman origin story? Warning: Spoilers for Joker follow.
Joker reveals it connects to the Bat-family early on. Arthur, the man who will be Joker, discovers his mother is obsessed with Thomas Wayne and his planned run for mayor.
For those who don't recognize the name, Thomas is Bruce's late father. Most fans know him as the man whose murder in an alley by an unknown assailant inspires Bruce to become Batman and clean up Gotham. But in Joker he's alive and on TV talking about how the poor of the city are worthless scum.
And according to Penny's letters, he's also Arthur's father.
Arthur decides to Thomas, to discover why he abandoned Penny, and maybe setting things to rights. To that end, he takes the train to Wayne Manor, only to happen upon its youngest resident.
Arthur's job is a clown who performs for children. Seeing little Bruce over the Wayne Manor wall, he goes into entertainment mode. He pops on a red nose, does some magic tricks, and hands his could-be half-brother a bouquet of flowers.
It's a short-lived friendship. Butler Alfred, still a young bruiser, storms down to protect his charge from the intruder. Alfred also tells Arthur his mother is lying. Penny never had an affair with Thomas. She was merely obsessed with him. Arthur was a child she adopted.
When Arthur confronts Thomas at a function not long after, he's given the same story. But who is telling the truth? The wealthy politician who's bastard could derail his career or Arthur's poverty-stricken mother? It turns out Penny is lying. She and Thomas did not have an affair, except in her deluded mind.
But though Bruce and Arthur are not related by blood, they are connected by fate.
The final scenes in Joker show Arthur embracing his insanity. He's inspired a legion of clown-mask wearing protesters after killing three rich young men on the subway. Now he stands up and willingly becomes their leader. First, he drives the mob to maul the cops on his tail, then he goes on The Murray Franklin Show and reveals himself as the murderer, before shooting Franklin in the head on live TV.
His actions set off a riot across the city, turning a scheduled protest into a bloodbath. Those caught up in the maelstrom including Thomas, his wife, and Bruce. They exit a theater into a mob scene, and in a panic, Thomas turns them into an alley in hopes of escape.
Batman fans know what happens next. It's not Arthur who does the deed, but one of his followers. As Joker takes the reigns and embraces his persona on one side of the city, on the other, Bruce stands in his family's blood, as the die is cast for him to become Batman.