All it takes is "one bad day".
The Joker escapes imprisonment and is planning to wreak havoc again. Batman goes out after him, trying to figure out what he's going to do next, but he struggles. Then a series of flashbacks tells us about the Joker's possible origin, from his beginnings as a loving husband and father, to his transformation to a crazed, psychopathic mania who only lives to prove that life is all but a joke.
Arguably the greatest comic book writer of all time, Alan Moore delivers a story that is told from the Clown Prince's perspective, how he sees the world, life and Batman. He also showcases the symbiosis between DC's marquee characters which had never been done before. Artist Brian Bolland is successful in bringing the panels and Moore's story into life, creating illustrations that takes insanity to a whole new level.
The book is hailed as one of the greatest Batman and DC graphic novels of all time and it really is in my opinion. I got emotional and psychological satisfaction after reading it. The symbiosis between Bats and the Clown is something to dig in, and more importantly, seeing the world in a villain's eyes - or a psychopath rather - is nothing but perfect. We always get to see hero's saving the day, talk about justice and the right way to do things, but we have never seen an anti-hero take his turn and show us his side of the story, what he believes in, and what his ideals are.
Plus, a long running, central DC character gets mutilated here, and that particular event actually became part of the official continuity, which shocked readers all over the world. Talk about controversial.
Then finally there's the joke with Batman doing something that I have never seen before. Trust me, you'll be surprised when you flip the last pages. Those were the most resounding and memorable comic book pages - especially the last one. Heh.
A little piece of factoid here: Heath Ledger said in an interview before that he was given a copy of this graphic novel as a preparation for his character as The Joker in 2008's summer blockbuster movie, The Dark Knight.
Maybe this whole Joker thing really "caught up" with him.