Hal Jordan thinks that in order to save the world he must tackle the insanity of the Joker. Little does he know that the madness is infectious. In an attempt to enable the Joker's redemption, Hal enters the twisted mind of the villain. To Hal's dismay, the touch of the Joker's skewed consciousness releases the darkest part of Hal's soul -- Parallax! Can Hal defeat his past evil self?
This issues ties into the "Joker: Last Laugh" crossover.