It's Ted's 31st birthday and he's been desperately spending the last few days trying to finish up the design for the rib place he's been designing. The guys planned a surprise party for him on the roof, but can't get him to leave the apartment. Ted ends up staying in and ends up in a fight with Lily's goat, who sends him to the hospital. The next day, Lily uses the goat as a metaphor and convinces Ted he can't keep chasing his architecture dream. Not everything goes according to plan. Ted decides to take the teaching job he was offered by Stella's husband, and tells his children that it was in that class he met their mother.
Barney, meanwhile, asked Ted for his blessing on telling Robin he loves her using a suit metaphor. When Ted gives Barney permission to go after his Canadian suit, Barney is about to tell Robin how he feels at the party when she stops him and says I love you first. He gets freaked out and tells her to have a nice party. Barney later finds out from Lily that Robin overheard Barney's talk with Ted and decided to pull a Mosby to scare him off. She didn't want things to invariably end up bad. When Barney goes to confront Robin with his new found knowledge, they each exchange their share of Mosbies to each other and end up kissing. Yay.
Marshall, in an effort to get Ted to come to the party, decides to take "the leap." The leap is a jump from their rooftop to their neighbor's rooftop (about 7 feet) where it's nicely landscaped and has the hottest of hot tubs. After years of chickening out, Marshall eventually does it and the whole gang takes the leap.