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....