It's Always Sunny in Philadelphia Season 4 Episode 7

After Charlie accuses Frank of crapping in his bed, Mac and Dennis get involved and stage an overnight observation to get to the bottom of this mystery in a case of "Who Pooped The Bed". Meanwhile, Dee, inspired by "Sex & The City", takes the Waitress and Artemis for a night on the town while Artemis figures out the subtleties of the poop mystery.