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.
On Stan's orders, Mary arrives at Peter's auto dealership to turn in her beloved-but-too-conspicuous Mustang and learns that newly-engaged Brandi has joined the sales team. She's a natural saleswoman, but her fledgling career and newfound confidence are soon jeopardized when six luxury cars go missing from the dealership, and Brandi's record puts her at the top of the Albuquerque PD's suspect list. To add insult to Grand Theft Auto, Peter's parents make it clear that they don't exactly trust their soon-to-be daughter-in-law's intentions, and the tension is driving a wedge between Brandi and Peter.
As she attempts to get to the bottom of the car thefts, enlisting the help of a witness with a background in the art of the steal, expert wedge-driver Mary clashes with new ABQPD detective Abigail Chaffee, who's perky, considerate, and likable—pretty much the un-Mary. But while Mary and Abigail won't be planning a slumber party any time soon, it appears that Marshall has much warmer feelings for the new detective. And there's more bad news for Mary: with Stan interviewing new candidates for the team, yet another cheerful new face may soon be arriving to ruin her day.