SPOILER WARNING: The following review contains major spoilers for "Batman: The Killing Joke," on sale now via digital download.
If you were to rely solely on YouTube clips to get an idea of what kind of movie "Batman: The Killing Joke" is, you would rightfully come to the conclusion that it's a faithful retelling of the classic graphic novel, albeit with some odd decisions regarding Batman and Batgirl's relationship.
However, you'd only be seeing clips of half the movie. Literally. The first half of the movie is an entirely original storyline that has nothing to do with the Joker, and very little to do with Batman, for that matter.
Alan Moore and Brian Bolland's "The Killing Joke" is considered the seminal, bedrock story of the yin and yang relationship of the Dark Knight and the Clown Prince of Crime. Published by DC Comics in 1988, the graphic novel utterly reshaped what a Batman story could be -- hell, it showed us what a superhero story could be. "Killing Joke" redefined the hero and villain as two sides of the same dark coin, forcing readers to recognize that Batman wasn't a mere good guy -- he was the guy who stood against the maelstrom, refusing to succumb out of sheer force of will. And, we saw that there was little separating Batman and the Joker, aside from one bad day.
Its reputation as one of the forefathers of modern superhero storytelling cannot be overstated. However, with societal shifts over the past three decades, parts of "The Killing Joke" don't hold up, with Batgirl's sexual violation and treatment as a "woman in a refrigerator" -- a term used to describe when a female character exists solely to suffer in order provide a male character motivation -- chief among them.
This storytelling device isn't limited to comics and has existed nearly as long as storytelling has. Helen's kidnapping at the hands of Paris led to the Trojan War; St. George saved an unnamed princess from the dragon at the spring; and James Bond has been motivated by more dead women than there have been Green Lanterns. But those examples all come from less-enlightened eras; this oft-repeated mechanic has come to be considered lazy and/or sexist writing. To be fair, the trope has grown to include other variations on the theme: men, children, or families can also be metaphorically shoved in refrigerators solely to further the plot of both male and female characters alike. Even when genders are flipped, the mechanic is often shallow, and devalues the stories that could have been told in its place.
So yes -- while "The Killing Joke" graphic novel is a staple of superhero storytelling, it uses the Women In Refrigerators trope to further along the story arc of the protagonist, Batman. In making the animated adaptation, the filmmakers were acutely aware of this critique, and attempted to rectify it for the modern age.
They failed. They did try, but they failed in some egregious ways.
On the surface, "The Killing Joke" is a story about the Joker and Commissioner Jim Gordon. The point of the tale is to highlight the subtle difference between the Joker and Batman, but in its telling, it's about Gordon's utterly unflinching character and the Joker's attempt to break him. It's Christopher Nolan's "The Dark Knight," twenty years prior.
The Joker breaks free from Arkham Asylum and sets in motion his latest scheme. However, this one's not about banks or mass murder -- he's got something far more personal in mind. While Jim is visiting with his daughter, Barbara, the Joker shows up. He shoots Barbara in her stomach, paralyzing her, and kidnaps Jim.
He brings the commissioner to an abandoned carnival he's outfitted with all manor of insanities: a demented side show, a twisted house of mirrors, and a funhouse through the depths of insanity. He strips Gordon naked, and has him led about on a leash by his minions, repeating his refrain over and over: When life doesn't make sense, stop trying to force it. Go mad instead.
The Joker's point is that all that separates normal people from sheer madness is "one bad day." After he has spent the night softening up the Commissioner's mind with freaks, jeers, and utter sensory shock, he brings it all home with his coup de grace: a funhouse ride featuring pictures of the brutal maiming, sexual molestation, and implied rape of Barbara, Jim's daughter. It's enough to drive any man mad, but Gordon knows the stakes, and who he's dealing with. He refuses to give in, and when Batman finally arrives to put a stop to it, he insists the Dark Knight does it "by the book." The Joker's ride into madness didn't work.
The two opponents battle, but after Batman's inevitable victory, he stops short of beating the Joker silly. He offers a hand of sympathy instead, hoping that understanding and compassion can win where fists and righteous fury have long since failed. Joker sadly refuses, admitting it's too late for either of them; they're locked into this for eternity, like two mental patients who can't even trust each other's mutual madness.