To infiltrate a drug cartel, police Inspector Chan Ka Kui (Jackie Chan, Rush Hour, Rumble in the Bronx) goes undercover in a Chinese prison. There, he earns the trust of Panther (Yuen Wah, Kung Fu Hustle), a cartel member, by breaking him out of prison. With the help of another undercover agent (Michelle Yeoh, Crouching Tiger, Hidden Dragon), they travel to Hong Kong and join up with Panther's gang. Ka Kui is accepted by the gang's leader (Ken Tsang), but his operation is jeopardized when Ka Kui's girlfriend (Maggie Cheung, Hero) accidentally reveals his true identity.