Evelyn pops over to tell Charlie and Chelsea that there's an opening for a June wedding at the hottest vanue in town, the Mayfair. Suddenly, poor Charlie who's been delaying setting a wedding date suddenly has one. He decides he needs to go out to grab and drink, and Alan wants to tag along to continue to rag on Charlie for his now sealed fate.
Soon the boys wake up next to a strange girl they met the night before without a recollection of the night. Charlie is left with only some horrible memory flashes and wearing Alan's underwar. The two take a cab ride home and Charlie manages to sneak into bed with Chelsea and gets away with it, except for his guilt.
The next day, he's thinking about telling Chelsea about what he can remember of the night, but Evelyn talks him out of it with a horrifying story involving her cheating on his second step-father with the entire circus. When he's about to tell Chelsea that night, he instead tells her of Evelyn's story.
The episode title, "Untainted by Filth" was used by Charlie when describing white roses for their wedding center piece.