Mary and Marshall try to persuade an FBI informant to join Witsec, but the man remains reluctant. Elsewhere, Stan tries to wrangle an ex-boxer witness suffering from dementia. Steven Weber guest stars.
Mary's new witness, a former hit man with a penchant for explosives, falls in love. Mary has high hopes for the romance, but her optimism quickly turns to concern after the man is dumped. He's also bullied by a local politician and considers taking drastic actions to vent his frustrations.
Mary helps a parolee and former bank robber stay on the straight and narrow and acclimate to life outside prison. Meanwhile, Marshall pitches in to help Mary with a witness, but the witness ends up preferring Marshall's help over Mary's.
Agent Faber pairs Mary with a new witness who was involved in a mortgage-fraud scheme for a gang. Mary distrusts the woman, but Faber also seems to have ulterior motives during the case: namely, pursuing Mary.
Marshall deals with feelings of inadequacy when working with his dad (Geoff Pierson), who visits Albuquerque to round up fugitives with outstanding warrants. Elsewhere, Brandi returns home with an odd explanation for her disappearance and a guest in tow.
A woman wants to escape from her family, who are heavily involved in organized crime, but a secret from her past puts her eligibility for witness protection in jeopardy. In the face of these obstacles, she turns to Mary for help. Laura San Giacomo guest stars.
A witness with Asperger's refuses to testify against her friend, frustrating Mary. Elsewhere, Marshall and Stan try to prevent the deportation of a key witness, and Brandi comes clean about her investment plan with Scott.
Mary helps her latest witness, a homeless man who's struggling with the program, find his birth parents. But the search could cause more problems than she anticipated. Elsewhere, Scott asks to borrow more money, raising a red flag for Brandi.
Mary keeps an eye on a teen witness whose separated parents are too distracted by their own issues to pay attention to their daughter. Elsewhere, Marshall thinks a witness may have betrayed his brother; and Brandi tends to Scott after he's attacked.
Mary must protect a priest who witnessed the murder of a stripper in the Season 3 finale, but she soon learns that the priest is obsessed with saving another woman from a similar fate. Meanwhile, Brandi moves in with Peter, Stan informs a dead witness' daughter about the truth of her father's absence and FBI agent Mike Faber returns.