How can you tell if someone is cheating on you? In order to learn the truth, you could always consult with your friends -- or your co-workers. On 'The Office' (Thu., 9PM ET on NBC), Michael (Steve Carell) suspects that something is up. He thinks that his new girlfriend Donna (Amy Pietz) is messing around with another man. He then makes the questionable decision to employ Mr. Dwight Schrute (Rainn Wilson) as a private investigator.

[Spoiler alert.]

In the end, Dwight isn't very much help (which isn't that surprising). It's Pam (Jenna Fischer) who find evidence that Michael's girlfriend is actually a cheater. Michael goes out to confront Donna in the parking lot. There, he finds out that Donna has a husband, which means that technically, Michael is "the other man." Michael is shocked and confused by this development. "...I'm the mistress?" he says.