Well, they steal a painting that is being transported from the Louvre to another museum via truck by using an elaborate plan that involves perfume, a convertible, a motorcycle, Joe breaking into the back of the moving truck, Jim hanging on to the underneath of the truck and sawing through the floor, and parasailing. Assuming that he didn't like for own children very much. OK, that's the background. The plot really kicks in when Jim and Joe steal a cursed painting from a weird castle at the behest of French collector. The collector betrays them for no particular reason and has his henchmen try to kill them.
Big boom follows.
With Joe thought dead, Jim and Cherie retire to Hong Kong and get married. Two years later, Joe shows up, confined to a wheel chair. It seems the cursed painting is now owned by Chow, so Joe and Jim decide to steal it from him to teach him a lesson. Even though Once a Thief features a good dollop of gun-fueled mayhem, this really is a comedy. Most of the heist scenes are pretty ridiculous, and there are lots of scenes where the three amigos doing cute things together, like eating at sidewalk cafes and celebrating birthdays.