It's Always Sunny in Philadelphia

S04E04 Mac’s Banging the Waitress

Charlie discovers that the waitress is involved with someone else. He turns to Mac, his so-called best friend, to dig up details about this guy and to confront him physically. To Charlie's surprise, he learns that Mac is actually the one dating the waitress. Meanwhile, Dennis feels crushed that he isn't considered Charlie's best friend and goes to great lengths to win back his favor.