Is Ted having second thoughts about designing GNB's new skyscraper because it would tear down a landmark, or because he's trying to impress a woman? Meanwhile, Lily shares a secret about Robin's new boyfriend with Marshall.

Marshall struggles to fire an incompetent employee, everyone tries to figure out whom Robin slept with Halloween night, and Zoey, the GNB skyscraper protester, finds a way to turn Ted's class against him.

Ted and Zoey's feud continues during a fundraiser at the Museum of Natural History. Meanwhile, Marshall confesses to Lily that he enjoys his job at GNB, and a prank by Barney and Robin may lead Barney to his real father.

When Ted leaves the bar early to prepare a Thanksgiving feast for his friends, the gang winds up partying all night with The Blitz, an old friend from college who has bad luck. As a result, Ted is forced to spend Thanksgiving with Zoey.

Ted goes on a boating trip with Zoey's husband, "The Captain." Meanwhile, Barney's "Mermaid Theory" prevents Marshal from having a nice dinner with Robin, and the narrator tries to recall the details of a fight between Lily and Barney.

Ted discovers that he is in love with Zoey after she tries to set him up on a date with her naive cousin, and he cuts off their friendship as a result. But little does he know that her marriage status has changed and she loves him too.

Lily goes to Minnesota to bring Marshall back, but he's not ready to leave yet. Meanwhile, Ted and Zoey's relationship gets off to a rocky start, and Barney searches for a female "victim" the night before Valentine's Day.

The Captain asks Ted to help find out who stole Zoey away from him, Marshall withholds sex until he does something to help the environment, and Barney is reluctant to admit that he has a crush on Robin's friend.

Despite Zoey still opposing the new GNB headquarters being built on the Arcadian Hotel site, Ted and Zoey's personal relationship is thriving. That issue is only one of the many disagreements in their life, which Ted likens to the two challenging each other mentally. On the other hand, he sees Marshall and Lily's relationship as being boring, as they agree on everything to the point of them almost melding into one person. Lily and Marshall, on the other hand, see their relationship as being supportive, the way that relationships should be, whereas they see Ted and Zoey's relationship as being too much work to the point of being mentally exhausting. But Marshall quitting his job at GNB to work in a non-paying volunteer position at the NRDC and Zoey managing to get the Landmark Preservation Society to agree to a hearing over the GNB project may change everything on both sides. Meanwhile, Barney seems to be taking Marshall leaving GNB the hardest. He admits to Robin that his anger regarding Marshall's departure has to do with a meatball sub.

When Marshall switches sides on the Arcadian dispute, his friendship with Barney is compromised and the gang is banned from the bar. Meanwhile, Ted finally finds the courage to confront Zoey about the same topic.

Ted is forced to make the final decision on whether he thinks The Arcadian should be declared a landmark, and his future with Zoey and Barney's future at GNB hinge on his decision.

In the aftermath of Ted and Zoey's break-up, Ted is having second thoughts. He is freaking out about an issue with the GNB headquarters, and historically when Ted freaks out about something, he turns to dating an old girlfriend if only for something comfortable and familiar, however bad returning to that relationship ended up being. When Barney and Robin find out that Ted is going to meet Zoey to get back together with her, they have to try and figure out where that meeting place is so that they can stop him. Beyond piecing together this puzzle, Barney and Robin's quest leads to them discussing their own relationship. Meanwhile, Lily has come down with a case of food poisoning from some soup. Marshall eats the soup three hours later, which means that his symptoms of excessive vomiting will start three hours later than Lily's did. Marshall has a job interview with a major environmental law firm in a few hours, and he is just hoping that he can make it through the interview without vomiting. Ted and Barney argue about which of them will have the pleasure of pushing the button when the Arcadian gets imploded. And one of these situation is the starting point for an imminent trip to the altar.

Ted wants to get a wedding guest he especially despises to leave, but Lily wants him to stay because she likes him.