It's Always Sunny in Philadelphia: Season 6, Episode 7

6 - 7: Who Got Dee Pregnant?

Dee's pregnant, and she tells the guys that one of them made her that way during a Halloween party that none of them can remember. So they try to piece the evening together, "Potemkin"-style, aided by Artemis (Artemis Pebdani) and the McPoyle brothers (Nate Mooney and Jimmi Simpson).