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.

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.

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.

When Ted receives an unexpected call from The Captain, the gang reminisced about their last awkward encounter with him.

It's weeks before Barney and Robin's wedding, and one of the many things they still have to decide is where to live. The one thing Robin insists is that they don't live at Barney's apartment. So it goes on the market. However, Barney doesn't want to sell it to just anyone, but someone who is worthy of what was and will always be in his heart his "Fortress of Barnitude". Robin has a slightly different idea, which isn't as easy to achieve as she would like when she and others learn more and more about with what Barney has equipped his apartment. Meanwhile, Lily hasn't spent much time of late with her family because of her job with The Captain. She seems to be at his beck and call, and doesn't seem to mind. Marshall understands, until it affects a planned evening out. So, Marshall decides to cheat on Lily with someone who Lily knows.

"The Captain" offers Lily a job in Rome, but will she take it? Meanwhile, Ted won't stop raving about the body of Barney and Robin's wedding planner.

The guys try to figure out where Lily went after her fight with Marshall and why. Meanwhile, after a talk with her mother, Robin is concerned that she is marrying her father.