While Barney prepares for his wedding, he and Ted reminisce about Punchy's wedding, which was ruined due to Marshall's drunk antics. Robin considers telling Barney she has feelings for him until Barney receives a call from Nora.
Marshall finally lands his dream job, but he worries that his new employer would rescind the offer because of some past videos showing him as a streaker – Beercules. Meanwhile, Ted is at a loss over whom to take to the Architects' Ball while Barney tries to convince Nora that he can be trustworthy.
Ted runs into his old girlfriend, Victoria, and tries to make amends with her since he cheated during their relationship. She reveals she's engaged to a man she met in Germany. Marshall and Lily make a bet with Barney that could force him to wear Marshall's ducky tie.
Robin is forced to undergo court-mandated therapy sessions after she assaults a girl who is out to ruin Barney's relationship with Nora. Robin becomes attracted to her therapist, Kevin (Kal Penn). Ted gets too involved in Lily's pregnancy.
Ted takes his class on a field trip and Barney tags along. Meanwhile, Marshall takes matters into his own hands when his boss treads too carefully in making a huge settlement with a major company.
The gang intervenes in Ted's business when he refuses to gather information online about a woman he is going to date with disastrous consequences. Meanwhile, Marshall and Lily decide not to find out the sex of their baby, but Barney soon tempts them to change their minds while setting up the baby's room. Kevin, Robin's boyfriend, paints the baby's bedroom.
Kevin helps the gang realize that their partners remind them of one of their parents, depressing and embarrassing Marshall and Lily. Meanwhile, Barney and Nora's planned romantic evening is ruined by a string of horrible events.
Ted runs into "The Slutty Pumpkin" (Katie Holmes) but realizes that she is not "The One" and actually has nothing in common with him. Barney learns he is part Canadian, to his dismay and Robin's joy, and Lily has pregnancy brain.
While the gang reminisces about Hurricane Irene, Barney tries to cut a deal with Marshall and Lily to get out of wearing the Ducky Tie.
Barney and Robin are afraid to break the news to Nora and Kevin that they cheated with each other and want to get back together, while Ted and Marshall get stoned together at a concert.
On Thanksgiving, Ted and Barney talk about a potential life-changing decision together by adopting a baby. Barney then steals a baby who turns out to be his brother's new baby girl. Robin tries to discourage Marshall and Lily from moving to Long Island to start their new life as a family. Robin reveals a shocking secret to Barney concerning their night together.
Robin receives bad news and decides to hide it from the gang. Marshall is trapped on the roof of his house in the suburbs and is blackmailed by a teenager to stay there.
After celebrating New Year's Eve, Marshall visits his father's graveside to carry on their tradition of tailgating. Meanwhile, Barney and Ted open a bar called "Puzzles," and Robin is forced to fill in for an inebriated news anchor.
Marshall and Lily move to Long Island and her father won't stop tormenting them about their house. Barney declares himself "New Leader of the Gang" now that Lily is not present to boss Barney around. The new and 'improved' gang embrace the 'new Lily and Marshall' (Lily's stripper doppelgänger and her boyfriend) with bad consequences.
Lily asks for her father's help when she and Marshall decide to throw a housewarming party. Marshall's boss gets under Ted's skin and the two nearly come to blows, and Barney puts the moves on a wacky divorcée.
Lily and Marshall invite Robin and Kevin on a Valentine's Day trip to Vermont. Lily and Marshall worry about scores concerning the pregnancy. Kevin and Robin use the trip to talk about taking their relationship to the next level. Robin admits to Kevin that she can't have children and he proposes. Meanwhile, Barney realizes he has finally met his match.
After Kevin breaks up with Robin, Ted confesses his feelings to her. Barney finds something private in Marshall and Lily's things. Marshall says something to Robin that he feels Ted loves her too much to say. Special appearance by Conan O'Brien as a background extra in MacLaren's.
Barney tries to convince Quinn, aka Karma the stripper (Becki Newton), to go out with him. Ted wonders what to do with the room now vacated by Robin, who has moved out to the suburbs with help from Marshall and Lily.
Barney and Quinn want to move in together, but Barney's friends do not like or trust Quinn and try to break them up. Robin and Ted fight over who deserves to sublet Quinn's apartment.
Ted, Marshall and Barney embark on another "trilogy time", where they get together every three years to watch the "Star Wars" trilogy and envision what their lives will be like in another three years. Meanwhile Barney and Quinn move in together.
Although Ted is happy with living alone at his new apartment, Barney tries to convince him about going out every night, attempting to distract himself from the disturbing thoughts of what Quinn would be doing at that moment. Lily has a naughty dream, but Marshall is not in it. Robin finally starts her stint as a news anchor at World Wide News.
Marshall panics when a baby shower indicates that the birth is close at hand and is taken on a road trip by Barney to Atlantic City where he gets completely and utterly drunk. Ted hallucinates seeing Robin while trying to get over her and decides to patch up their differences. Barney tries to talk Quinn out of being a stripper. Lily goes into labor.
When Lily goes into labor, Ted and Robin distract her by telling her stories while Marshall hurries back from Atlantic City.
Ted is conflicted when Victoria approaches him on her wedding day and asks him to elope. Meanwhile, Barney and Quinn are arrested by airport security while on the way to Hawaii when Barney refuses to open a box they are travelling with.