Lily gets asked to be in a terrible play, and the gang goes to show their support. While Ted, Marshall and Robin all give her props, Barney keeps it honest. Lily is upset because of his honesty, and says she would have been nice to him no matter how bad a play he was in. This sets up a classic ending.
Meanwhile, robin finds out where many of the things of Ted's came from: past girlfriends. Feeling insecure, she starts seeing his ex-es everywhere they left something. Ted gets rid of it all before learning that Robin's dogs are all love gifts from her past boyfriends. Unable to cope with this new bit of info, Robin does the same for Ted, and gets rid of her dogs. Only to find out that Ted didn't throw anything away, which led to a large fight. The end of the fight? Ted and Robin are moving in together.
Then the gang finds themselves in front of the stage, to view Barney's play, which is specifically created to be terrible. The episode ends with Marshall no longer being able to take it, and bringing out number 2 of the slap bet: he slaps Barney to the ground.