The unnamed double is the mistress of pilot Stefan Chabrol, and a dead ringer for his wife, Barbara. In the opening scenes of the episode, the mistress (whose name is never revealed) enters the restroom and encounters the real Barbara, and asked for her lipstick because it was the only thing she could not match. The mistress turned heel and shot Barbara to death with a silencer, and afterwards, the evil double put on Barbara's lipstick and met Stefan, while posing as Barbara.
Monk, who is on a plane with Sharona on their way to meet her aunt, notices something amiss when he sees Stefan and Barbara. The first time they were shown, the real Barbara had to stand on her toes to kiss Stefan. The double does not. It is later revealed that the real Barbara Chabrol (née Lesser) is a trust fund baby; her family owns Lesser Pharmaceuticals. The mistress killing Barbara was part of Stefan's plan to continue living off Barbara's riches, by passing her off as Barbara. She would later trap Monk in the lavatory while Stefan poisoned Bernard, an old friend of Barbara's. After Barbara's body is found by Randy Disher, who Monk called about the case, the evil mistress was arrested along with Stefan.
- Jennifer Dale later played villainess Evelyn Whitland in 2019's Homekilling Queen.