This is a story of three orphans. The man who raises them an who they call as father teaches them to be thieves. They have also another father figure, a police who helps them and try to get them straight. But years later these three, Joe (Chow Yun Fat), Jim (Leslie Cheung) and Cherie (Cherie Chung) are art thieves. They travel around the world and steal worl famous, heavily guarded paintings. And the nevet get caught, they are professionals. But now they want retire anyway, when one more challange comes. It's very difficult one, but they decide to steal that one last painting. But things begin to go wrong. Once a Thief begins as a comedy, They are fooling around and having some fun. Joe and jim become kind of competirors when they both fall in love with Cherie. More action occures during the supposed to be the last painting to steal. And in the end story turns a heroic bloodshed fight, when their evil 'stepfather' shows his real faces.