After his best friend Marshall proposes to his long-term girlfriend, Lily, Ted solicits help from his friend Barney to find 'The One' for his life. He manages to get a date with Robin, a girl he met at his usual neighborhood bar, but threatens to scare Robin away when he accidentally reveals his love for her on the very first date. Meanwhile, Marshall accidentally hits Lily in the eye with the champagne stopper after they get engaged, forcing her to wear an eyepatch.

In an attempt to repair his situation with Robin, Ted instead pursues a 'casual' relationship with her by inviting her to a series of parties. Marshall tries to write an important 25-page law paper, but Ted's parties and Lily's post-engagement desire distracts him. Meanwhile, Barney tries to end a relationship he unknowingly started.

Ted agrees to let Barney disrupt his routine by taking an impromptu trip to the airport with him that eventually leads the duo to Philadelphia and trouble with airport security. Meanwhile, Lily and Robin go out for drinks, but Lily becomes jealous when she is not as successful with men as Robin is, for which she blames her engagement ring. Marshall travels between both situations in an attempt to rectify the group's problems.

Ted's continuing search for 'The One' leads him to look into his past to rekindle old flames. Unfortunately, the woman he is thinking of is the same woman he broke up with years ago on her birthday, and she still holds a grudge. Meanwhile, Barney dares Robin, the anchor of Metro News One, to slip questionable words or phrases into her daily newscast in return for money. Despite this, Robin begins to appreciate the importance of her job more.

Robin invites the group to a nightclub, but Marshall and Lily decide to host a wine and cheese party in an attempt to act more mature. Marshall and Lily quickly grow bored, however, and escape their own party in order to join the rest of the group at the nightclub, where they discover Robin outside, unable to gain re-entry into the club. Meanwhile, Ted and Barney experience unforeseen events while dancing.

Marshall and Lily are excited to participate in the couples costume competition at their usual bar. On the other hand, Robin's refusal to dress in a couple costume with her boyfriend puts stress on their new relationship. Meanwhile, Ted makes his annual visit to the rooftop Halloween party in search of a girl that he met there years ago.

Robin directs a skeptical Ted to an online matchmaking service with a 100% success rate. When the service gives Ted zero matches, Ted sneaks information out of the matchmaker's computer to visit an engaged dermatologist in an effort to debunk the prediction that there are no women out there for him. Meanwhile, Marshall and Lily become obsessed with chasing a strange and unusual critter out of their apartment.

When Lily decides to formally move into Marshall and Ted's apartment, Barney warns a skeptical Ted that the couple will eventually force him out. When Ted discovers that Lily replaced his beloved, but beat-up, coffee pot with a brand new one, he begins to think that Barney is right. Meanwhile, Barney invents a new dating law and tries to convince others to use it.