It's Always Sunny in Philadelphia Season 3 Episode 10

Mac Is a Serial Killer

After Mac arrives to the bar with scratches on his neck and exhibiting strange behavior, Frank comes to the conclusion that he is the serial killer terrorizing young women in Philadelphia. Frank sets out with his trusty chainsaw and a doubtful Charlie to find evidence to implicate Mac in the murders. In search of another suspect, Dennis and Dee do a little role-playing in order to help themselves better understand who they might be dealing with.