Hong Kong Director Dante Lam delivers thrills aplenty in this nimble action-horror hybrid. It's mostly cops and robbers, but yes, there's also that matter of That Demon Within. In fine form, Daniel Wu plays a fresh-faced but mentally fragile young constable, stationed on the ER night shift, who gives blood for an emergency transfusion only to learn the life he saved was that of a ruthless cop-killer (Nick Cheung). Nightmares ensue. The rescued patient, understandably known around town as the Demon King, escapes the hospital and rejoins his gang of thieves to continue a spree of deadly violence, leaving the young officer with PTSD of supernatural proportions, along with some major guilt. Reportedly the film was inspired by the case of an actual Hong Kong cop who murdered several fellow officers and finally was gunned down by several others. It's always bad news when the demon gets out.