Pitt stars as Jerry Welbach, a small-time loser who is given no choice but to run an errand for a powerful boss (Bob Balaban) who will have him killed if he fails. But if he accepts the job to go to San Miguel to pick up the beautiful handcrafted gun known as the Mexican, his loud, demanding girlfriend, Samantha (Roberts), will leave him and move to Vegas. But through a course of bizarre events, his contact is shot in the top of his head, the gun is stolen, and Sam is kidnapped and held hostage by a hired killer (James Gandolfini) who is not all that he seems.