Mary Shannon's (Mary McCormack) friends and family think she's just a glorified courier, but she's actually a U.S. marshal working for the Witness Protection Program. When witnesses come forward to testify against the worst-of-the-worst criminals, the U.S. marshals protect them before and after they testify. Whether they're career criminals or just people who had the misfortune to witness a crime, they all share one thing in common: somebody wants them dead.