Hong Kong police officer Kit (Wu Jing) goes undercover in order to catch Mr. Hung (Louis Koo), the mastermind behind a crime syndicate. When the operation goes sour, and the undercover cops are betrayed, Kit disappears without a trace. Uncle Wah (Simon Yam) defies the order from his commanding officer and tracks Kit to a prison in Thailand. Thai Police officer Chai (Tony Jaa) becomes a prison guard in order to raise money for his daughter who has leukemia. He’s assigned to keep an eye on Kit. Even though Chai and Kit are in opposing positions and they don’t speak a common language, Kit turns out to be a suitable bone marrow donor who can save Chai’s daughter. While Chai is determined to keep Kit alive, the warden Ko (Zhang Jin) wants him dead to ensure the smooth operation of the prison, which is the front for Mr Hung’s organ trafficking business. Mr Hung shows up in Thailand so he can use his younger brother’s (Jun Kung) heart in a heart transplant to save his own life.