The GNB tower opens tonight and Ted is nervous, but Marshall and Lily say it's going to be amazing. They're going to be there to toast him, but they're going to have to leave a little early because Mickey is giving them a night away from home. They've planned the whole outing down to the minute. Ted says they can leave early and reveals that Barney isn't even going to be there. Robin asks why, but Ted doesn't say. Marshall and Lily take off and Robin asks Ted again the real reason Barney isn't going to be there. Faced with a chance, Ted tells her Barney didn't say what he was going. Ted asks Robin if she wants to be his date to the opening.
Marshall and Lily are leaving Mickey with Marvin, and Marshall goes through a long rendition of the bedtime song he and Lily sing to him. Ted calls Marshall, who says Ted needs him. Lily says the news better be huge. "That's huge" Marshall says. Ted has told Marshall, and then admits that instead of telling Robin, he asked her to be his date to the opening. Ted doesn't even believe why he still thinks he has a chance with Robin. He says he needs to grow up, then adds, "By the way, I'm breaking a jinx swear here, so don't tell Barney or he gets to whack me in the nuts three times with a whiffle ball bat". "Sure" Marshall says, with a seriously look on his face. Ted says Robin deserves the chance to go after Barney one last time, if that's what she wants.
Ted thanks Marshall and says he's about to go tell Robin. Marshall calls Lily, who's just checked into a hotel, and tells her he's going to be a little longer. She tells him to hurry or she's going to start doing Number 11 on their list by herself. Marshall tells Ted he still thinks he could have a shot with Robin. Ted thinks it's selfish to not give Robin the chance to go after Barney. Marshall tells Ted he won't leave the bar until Ted promises he will not tell Robin. Lily calls Mickey to check in on Marvin, and Mickey says everything is going just fine and that Marvin's head smells like love. Lily is sad. Ted picks up Robin, who looks stunning in a red dress. Ranjit is back.
He's driving them and gives Robin a "Hubba hubba" Ranjit asks what the special occasion is and Robin tells Ted to make the announcement. "Barney is getting engaged" Ted blurts out. Robin is stunned. Ted tells her that Barney asked him to keep it a secret, but he thought she would want to know in case she wants to do something about it. He asks her if she does, and Robin doesn't answer. Marshall and Lily show up to the GNB tower opening, and Lily is drunk. They start to miss Marvin. Robin tells Ted she's not chasing after Barney any more. She insists she's not in love with him.
Ted tells her about how it's going to happen on top of the WWN building, because that's apparently Patrice's favorite place in the city. Robin says that's her favorite spot in the city and yells at Patrice. She admits that one detail stings, but says there's no place she'd rather be than at Ted's building opening. She tells Ranjit to drive there. Marshall and Lily end up crying in the corner while looking at a picture of Marvin and singing his bedtime song. Robin tells Ted, while they drive, how "huge" it is that something he thought up in his head is now part of the New York skyline. She tells him it's huge and he's "the star of the party".
She adds that the future Mrs. Ted Mosby might even be there. Ted says maybe. The car stops and they're not at Ted's building. They're at the WWN building. Robin insists that they go to Ted's party, and he asks her if she wants to follow her heart instead of making small talk with bankers. Robin says she can't go on making an ass out of herself, and Ted gives her a defense a making an ass out of one's self. He says that eight years ago he made an ass out of himself chasing after her, and he's made an ass out of himself several times since, and it's given him a great friend. He finally convinces her to get out of the car and get up on that roof. Robin opens the door to the roof and sees rose pedals spread out and candles lit.
She picks up a piece of paper. It's titled, "The Robin" from his playbook. He details, step by step, all of the steps he's taken. Robin sees Barney and tells him that after lying and manipulating her, she can't trust him. She thanks him for setting her free, because she can't be with a man who thinks this "trick, this enormous lie could ever make me want to date him again". He tells her to turn over the page. Step 16 is written: Hope she says yes. "Robin Scherbatsky" Barney says, on bended knee and with a ring in hand, "will you marry me?" "Yes" she says.
They kiss. Ted's building is dedicated and he gets a toast and shakes hands. Marshall and Lily go back home to see Marvin. Ted goes to a window high atop the GNB and looks out somberly. The episode ends at this point.