In Gotham City, where heinous acts of violence are a daily occurrence, the GCPD relies on Harley Quinn, a young forensic psychiatrist and profiler, to consult on their toughest cases. But Harley is haunted by one unsolved case-the night she discovered her roommate's body marked with the signature of a notorious serial killer known as The Joker.

Five years later, the case remains unsolved and a new series of horrific murders occur throughout the city. As the murders escalate, Harley's obsession with finding the depraved psychopath responsible leads her down a dangerous path. When the past and the present finally collide, Harley has to decide how far she is willing to go-and how many lines she is willing to cross-to solve these cases once and for all.