A shootout on the night-time streets of Hong Kong results in a bleeding man being taken to hospital. Reticent policeman Dave doesn't think twice about donating blood to save him. But the wounded man turns out to be Hon Kong, head honcho of a brutal group of street-thugs known as the ‘Gang from Hell’. The gang commit murderous hold-ups wearing demon masks and show no mercy to either civilians nor the police. Dave is plagued by a guilty conscience and decides to dismantle the gang. He is haunted by increasingly dreadful visions which appear to be connected to his past. When Hon is betrayed by gang-members, Dave decides to throw in his lot with the criminal in order to achieve his goal. But boundaries become blurred: Dave falls under suspicion of murder, colleagues turn out to be corrupt and his visions become ever more threatening ... In That Demon Within multi-award-winning action specialist Dante Lam, who with this new film is now a guest of the Berlinale for the fourth time, tells a virtuoso, psychologically ambiguous cop story imbued with the shady atmosphere and extremes of the megacity.