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Nightmarish acts of violence in Gotham City are a daily occurrence. Harley Quinn, a forensic psychiatrist and profiler, consults with the PGDP on some of the worst cases. However, Harley is focused on an unsolved murder that has haunted her since she found her neighbour's body bearing the signature of the notorious serial killer known as the Joker.

Five years later, the case remains unsolved, but a new series of gruesome murders is sweeping the city. As the number of murders rises and the perfectly crafted crime scenes become more elaborate, Harley's obsession with finding the psychopath responsible leads her down a dangerous path. As past and present finally collide, Harley must decide how far she's willing to go - and how many lines she's willing to cross - to close the cases for good.