To those that know her, Mary Shannon is a glorified meter maid, but her real job as a federal marshal is to oversee federal witnesses who have been relocated to Albuquerque through the program. She has to balance her job as a marshal and the secret identity she must use to hide her high-risk, high-impact job from her family.

Mary Shannon is a U.S. Federal Marshal with the highly secretive Witness Protection Program. For Mary, living a double life and getting shot at on a regular basis are all in a day's work. So are teaming-up with her trusted partner, Marshall Mann, to protect Federal witnesses who are being relocated to Albuquerque. Mary does all this while juggling a complicated personal life.

In this final season, U.S. Marshal Mary Shannon returns from maternity leave to her job at the Witness Protection Program, and must quickly find a way to balance motherhood and her job protecting people, both the innocent and cold-blooded killers alike.