From the graphic novel of the same name by Brian Azzarello.
This is a really good origin story for the Joker and also tells how Barbara Gordon goes from Batgirl to being Oracle, the (basically) reference librarian/computer whiz that Batman and many other superheroes in the DC universe rely on.
Sure, there are things we’ve seen before- Batman trying to talk someone else out of doing exactly what he himself does (being a vigilante), or the chasing-the-bad-guy-through-the-funhouse thing, where of course the mirrors reveal more than just the reflection…but given who the Joker is, this is actually very appropriate, and works well.
But the writing and animation are both good, the buildup is excellent, and there’s more character work here than one generally finds in a comic-book movie, including some great backstory on The Joker.
The storytelling here is good, but (and Azzarello’s name on top should be enough warning) it is adult. There is strongly implied sex and a probable rape scene, and of course Barbara is shot in the spine and loses the use of her legs, hence Oracle.
Better than average for an animated feature made from a graphic novel!