S1, E9: Internal Affairs knows Rico Eps is one of the dirty cops on the Chicago force who isn't above planting evidence to make a case. But instead of taking him down, IA offers him a deal in exchange for investigating his fellow cops. Eps takes the deal, and when his partner finds out, he's almost killed. Staying on the police force obviously isn't an option. Eps has to leave everything behind and enter Witness Protection. When Mary and he meet there are immediate sparks, and even though she knows it's against the rules and an all-around terrible idea, she gives into temptation and sleeps with him. Afterwards, they part ways and have minimal contact over the next six months. Things might have continued like that, but when an Albuquerque cop with ties to Chicago is murdered, Eps quickly becomes the prime suspect. Mary has to both prove his innocence and keep their past from damaging her career.