Mac makes a tape of Project Badass and shows it to Dennis, who is impressed. Mac tells Dennis that he saw Charlie smashing his copy of the tape. Charlie enters and asks Mac -- his best friend -- to help him track down the man that is banging the Waitress. Dennis grows jealous that he is not Charlie's best friend. Mac visits The Waitress' apartment and tells her "he's on to us".
Charlie heads over to Dennis and Mac's apartment to camp out while Mac is on the manhunt and Dennis grows increasingly needy to have Charlie as his best friend. When Mac radios in that he beat up "Don Johnson", Dennis plants a seed in Charlie's head that Mac is the one banging the waitress. Mac gets home late and a severely drunk Charlie acts standoffish and heads to sleep in Dennis' bed. Mac tells Dennis while Charlie sleeps that he is really Dennis' best friend and that he is just banging the Waitress to get back at Charlie for destroying the tape.
The next day Mac and the Waitress steal Dennis's sex tape of him banging the Waitress. She then tells Mac he can bang her if he takes her to Charlie's apartment. Dennis and Charlie overhear the conversation on their walkie-talkies so they hide under the blankets on the bed. Mac, dressed as Charlie, finds them. As the three friends argue the Waitress searches for Frank's sex tape with her. Charlie tells her he destroyed it and she leaves, with only a mere "thank you". Dennis apologizes for trying to sleep with the Waitress and the three guys agree to all be best friends together.
Mac makes another episode of Project Badass and Charlie and Dennis talk to each other how they think Mac is in love with both of them.
The episode title, "Mac's Banging the Waitress", is actually not quite accurate. Despite numerous attempts, Mac actually doesn't bang the Waitress.
Title of this episode also could be considered as some sort of prank from a stag-party - because two days after it was first aired, on September 27, Rob McElhenney and Kaitlin Olson are will have a wedding. That's also explains why Sweet Dee (as well as her father Frank) is out of this episode.
Only four characters are been involved in this episode - that's Mac, Charlie, Dennis and the Waitress.