Taking place immediately after where the previous season left off, Season 9 covers the events of a single weekend that lead up to Barney and Robin's wedding. During the course of the weekend, "The Mother" is separately introduced to Robin, Barney, Lily, and Marshall, before finally meeting Ted. The season also features frequent flashbacks and flashforwards in order to fully integrate The Mother's character with the rest of the cast.
On their way to Long Island for their wedding weekend, Robin and Barney come across a startling family discovery. Meanwhile, Marshall sees something online that changes the course of his journey east, while Lily confronts Ted about letting go of Robin.
When James reveals that he is getting divorced, Robin worries about how Barney will take the news. Meanwhile, Marshall races to find a way to get to New York in time for the wedding, and Ted deals with being alone at a romantic hotel.
When Lily discovers Ted’s list of things he wants to do in New York before he moves for Chicago, she becomes frustrated that she is the only person who knows about his plans. Meanwhile, Robin and Barney realize they have very little time together before their relatives arrive for their wedding.
While Barney works through his anger at Ted after finding out he still has feelings for Robin, Marshall Skypes in to act as a judge and help settle their dispute. Meanwhile, Lily realizes she is Robin’s only girlfriend.
When Barney is forced to pick sides in a fight between Robin and his mother, Lily coaches him on how not to ruin his relationship. Meanwhile, Ted and Marshall get into a dispute over wedding gifts and thank-you notes.
When Ted finds himself with three prospects to be his date for the wedding weekend, he chooses poorly. Meanwhile, Barney and Robin have a confrontation with their minister, and Marshall learns more about his driving companion, Daphne.
When Daphne sends a troubling text message to Lily, Marshall enlists the gang’s help to remove the message while enforcing the “No Questions Asked” rule.
When Robin and Loretta's conflict escalates, Barney is caught in the middle. Meanwhile, Marshall and Daphne deal with a stowaway on their road trip, and Ted and Cassie try to enjoy a trip to a lighthouse.
In a flashback episode, the gang ponders the future of the Ted, Robin and Barney love triangle. Meanwhile, Barney accepts a series of challenges from Lily and Robin.
When Barney’s father arrives at the hotel, Barney devises a plan that makes Robin very nervous. Meanwhile, Ted is convinced someone framed him by messing up a wedding-related duty.
Marshall tries to get Marvin to sleep by reciting three rhyming bedtime tales about his friends as they wind down their epic cross-country journey.
Barney is shooting to have the rehearsal dinner at a laser-tag venue, while Lily is upset when Ted fails to live up to a promise.
The gang encounters a guy at the wedding who intentionally stirs up trouble among them, and Marshall finally arrives at the Farhampton Inn.
In an attempt to deliver a devastating final slap to Barney, Marshall explains how he mastered the Slap of A Million Exploding Suns.
When Barney drinks too much, Ted and Robin take advantage of him and get him to reveal secrets he’s been hiding for years. Meanwhile, Marshall does everything he can to avoid fighting with Lily.
On the 200th episode, The Mother recounts how she met Ted and what she’s been up to for the past eight years.
Robin and Ted reminisce about past relationships as they search for a drunken Barney on the day of the wedding. Lily and Marshall come to a resolution. Barney befriends two men who are eager to meet women.
When Barney suffers from the world’s worst hangover the morning of his wedding, the gang tries to figure out the far-fetched ingredients to concoct the Stinson Hangover Fixer Elixir.
On the morning of the wedding, Lily and Robin have a fight and Barney freaks out about which suit to wear. Meanwhile, the gang watches a movie Ted has forbidden them to see.
Marshall enlists Ted and Barney to help him figure out where Lily went when she left in the middle of their fight.
Gary Blauman shows up at Barney and Robin's wedding, sending the gang into a frenzy about which table to seat him at. Through their debate, they realize each of them either hates or loves Gary. Meanwhile, in the year 2014, Ted and The Mother go out on their first date.
With only a half-hour to go, both Barney and Robin have panic attacks about their upcoming nuptials. Meanwhile, Marshall and Lily rewrite their old wedding vows.
Ted finally finishes telling his kids the story of how he met their mother.