Harley Quinn and Joker try to ambush James Gordon at the dentist office but Batman gets the drop of them and even mocks Joker for being to predictable. They get away but Joker's pride is wounded and Harley can't even get a smile from him. She is eventually tossed out with the hyenas and she blames Batman. She feel Batman gets in the way of the two and if Batman was out of the picture she could have Joker to herself.
In a series of flashbacks we see the origin of Harley when she went by Harleen Quinzel. He gymnast scholarship to Gotham University and how she at the very least seduced her grades from her professors and graduated. Ending up at Arkham Asylum She meets and eventually falls in love with the Joker and even breaks him out as Harley Quinn.
Harley hatches a plan to capture Batman. She is meticulous over every detail and rechecks everything until Batman wakes up hanging upside down over a tank of piranha. Finally going to kill Batman Harley hears Batman tell her how mad the Joker would be if she did it without him. Convinced she calls joker and he gets there as fast as possible.
When he arrives he in in a rage that she had done this and that she didn't do it right. Joker pushes her out the window and nearly kills her and then he lets batman away...kinda. The two fight and Batman tells Joker Harley did better that the Joker ever did and that it was his ego that let him go. The two fight until Joker falls and disappears.