Frank spent literally the entire episode in just his underwear, trapped in some sort of plastic playground apparatus. And it's one of the reasons -- if not an absolutely fundamental reason -- that Always Sunny keeps ticking nine years into its run. Other comedies of the same satirical persuasion (say, South Park) can get caught up in a game of cleverness escalation, where each new season requires the comedy be more pointed or obtuse. I'm not saying it's bad on South Park. But it's a wonderful quality of Sunny, this acceptance -- embrace -- of abject stupidity over all else. A main character was stuck for 21 minutes in a piece of playground equipment. And he didn't get out at the end.
Everyone else got caught up in a pyramid scheme initially proposed -- or, well, disseminated -- by Dee. The product was "Invigaron," an energy drink. Deriving its power from berries, which Charlie's very impressed with, the drink is meant to reduce stress levels. Do you have high stress levels? You should drink Invigaron.
Sickened by their easily duped friend(s) and determined, naturally, to prove that they themselves were not dupes, Mac and Dennis set out to prove they could weather the Invigaron pitch without getting sucked in.
They left with a timeshare, and three weeks of annual pre-paid vacations.
Eventually, everyone realizing that they'd gotten duped one way or another, the gang attempted to enlist solider Ben (one of Dee's old boyfriends) to fall prey to their operation. But even he couldn't make a move that stupid! One final plea to Invigaron resulted in Mac, Charlie, Dee, and Dennis paying to break their contracts (being led to believe this is some great loophole). Their assumption at this point? That Frank, somehow, was the mastermind of the whole operation.
Over credits: nearly naked Frank, still stuck in the playground fixture, howling at the throngs of children playing all over him.