US Marshal Mary Shannon works for the Federal Witness Protection Program (WITSEC), a witness protection program that provides hiding places and protection for professional criminals, pathological liars, thieves, murderers and the occasional innocent bystander. During the long work hours, she can count on her know-it-all partner Marshall Mann and her trusty, dilapidated cart. In private, it is mostly clearing up the mess, with an on-again, off-again relationship, an unstable sister and a mother who mainly works on her system.