The episode takes place while the gang is at McLaren's and is told primarily through flashbacks. Robin has been busy with her new show (which airs at 4 am!!) and wants the gang to catch her up on what's been going on.
Ted reveals that his ex-girlfriend, the pretentious Karen (guest star Laura Prepon) has moved in to Town. Then the rest of the episode mostly is about how terrible she was for him, having cheated on him multiple times in his own bed when she came to visit him in college. Ted would always walk in the flashbacks to some meat head who would say, "Sorry, Bro" to him. This time, however, it turns out Karen is cheating on that meat head with Ted and he got to deliver the "Sorry, Bro." He surprises the whole gang when they find out he's dating her again and she shows up at the bar in the end.
Meanwhile, Barney makes Marshall tell his embarassing story about work. Apparently Marshall forgot to bring pants to work and only brough his workout shorts so after the gym he had to call Lily to drop some off for him. When Barney received the pants for Marshall he cut them into little cutoff suit pants. In the end of the episode, Marshall reveals he's still wearing the cut offs because he likes how they make his calves look.
Other funny bits from the episode include Robin's sleeping pill hallucinations and the gang telling stories about lunches with exes.