In Plain Sight Episode 5.03 Reservations, I've Got A Few

WITSEC's latest witness is Eddie Fredericks/Williams, a former Navy Seal who, along with his partner, Bo, has been pretending to be an FBI agent in order to help track down individuals for an Indonesian warlord. After the FBI catches Eddie, WITSEC agrees to protect him while he serves time, but things go awry when Eddie escapes from the WITSEC office - on Marshall's watch. Using Delia's car, Eddie makes his way to a nearby reservation. When the team finds Delia's car, it's burned and Eddie is gone, so Mary, Marshall and Heather, a Native American detective who seems to have a thing for Marshall, head out on horseback to find Eddie. The search lasts longer than expected, so Marshall has to cancel plans with Abigail and Mary must ask Mark to babysit Norah for the night.

Although horseback riding is not Mary's strong suit and Marshall feels guilty about the situation, the group still manages to keep safe and get the job done. And lucky for Marshall, Stan is able to contact Associate Director Campbell to make sure that Marshall doesn't get in trouble for his mistake.