Mary suffers the consequences when an attractive witness, who she slept with, is accused of murder.

A northern California marijuana mastermind cannot testify without using the drug to neutralize his anxiety disorder.

Mary and Marshall find themselves in a difficult position when Stan's very first witness turns up dead and he is implicated in her murder.

Mary lends her expertise for the U.S. Marshalls by making a training video with a film crew. Elsewhere, Marshall prepares the arrangements so that an elderly mobster in Witsec can go to his son's funeral.

When a Wall Street blue blood rats on his boss' investment fraud and hides out in WITSEC, something from his past life comes calling.

Mary’s new witness falls in love for the first time; Mary must stop her witness from taking out his pain in the form of an explosive device.

A witness with Aspergers who has no social filter and can't lie stymies Mary, particularly when the witness refuses to testify against her only friend – the key conspirator in a numbers case. Meanwhile, Marshall and Stan race to figure out why an Immigration and Customs Enforcement agent is holding up the visa of a key witness before she is deported. Plus, Brandi reveals her plan for the money she made by investing with Scott.

Mary uses a thief to catch a thief when she asks a witness to consult on a crime in order to exonerate Brandi of grand theft auto.

An Amish witness comes into the program and must learn to adapt.

Mary sees whether she can balance motherhood with a demanding profession and a dysfunctional home life, or if she will put her baby up for adoption.

When Marshall's witness escapes from the WITSEC offices and disappears on an Indian Reservation, Mary and Marshall will need the help of a Native American detective to track him down.

Mary is still reeling from the flood of emotions stirred-up after having to arrest her father, James, when he unexpectedly showed up on her doorstep after being AWOL for 30 years. Bad goes to worse when Special Agent Robert O'Connor, Mary's "favorite" FBI agent, demands her assistance to get James to provide critical information regarding his old partner-in-crime, Cormack “Sully” Sullivan, one of the FBI's 10 Most Wanted.

Series Finale: Drastic changes take place at the Albuquerque WITSEC office. Mary finds herself caught between a witness and an unexpected family reunion.