Joker and Lee (Harley Quinn) set off a bomb during Arthur's trial. Arthur flees after the blast, aided by his supporters. Eventually he reaches the stairs where Lee breaks up with him (as she realizes he’s not the joker, but his true self is Arthur) and then the police arrests him and sends him back to the asylum. Here a fellow inmate, comes to Arthur, to tell him a joke but instead of the punch line, he stabs him with knives in the stomach. Arthur is left to bleed and die and this fellow laughs psychotically, and gives himself the Joker/Heath Ledger style Glasgow smile with the knife. Movie ends. "That’s Life" cover by Gaga plays.
As if we couldn't see in the trailer for this that the Joker was going to be a supporting actor in his own sequel...
Spoiler. Spoilers. Spoilers.