Harleen Quinzel was a brilliant and successful criminal psychiatrist. After encountering the criminal mastermind, The Joker, at Arkham Asylum, Harleen develops a fascination with the madman's maniacal psyche. She begins to treat him, only to find herself completely under The Joker's control. Unable to break his hold on her, Harley enters a world of crime as The Joker's criminal accomplice and lover, Harley Quinn.
Harley quickly becomes one of the Dark Knight's deadliest enemies, as she carries out The Joker's every evil whim. She develops a villainous friendship with Poison Ivy and the two become sisters-in-crime by virtue of their shared hatred for male domination in the criminal underworld.
Although her true personality has been subjugated by The Joker, beneath it all Harley remains a sweet lovable person. Upon her arrest, she is deemed mentally unbalanced and sent to Arkham Asylum - not as the brilliant doctor she once was, but as one of Gotham's criminally insane on the other side of the padded cell.