That Demon Within (Mo Ging, traditional Chinese character: 魔警) is a 2014 Hong Kong action psychological thriller film directed by Dante Lam and starring Daniel Wu and Nick Cheung.

Reclusive cop Dave Wong (Daniel Wu) unwittingly saves the life of criminal gang leader Hon Kong (Nick Cheung) by donating his blood to him.

During psychotic episodes, Dave experiences his own demons within as he sets out to play off the gang members against each other, resulting in everyone's annihilation.

As a young, innocent soul who had lost his father, he impulsively went on to take revenge, resulting in excessive guilt that predisposed the onset of psychosis later in his life.

The event whereby he rescued Hon Kong reminded him of his suppressed memories as a child who had done wrong but trying the hardest to make reparation.