Barney begins the season with the word, "-dary!" Robin returns from a trip to Argentina with her new boyfriend, Gael (Enrique Iglesias), and Ted must adjust to life as just her friend, while watching Robin and Gael fawning over each other. Marshall and Lily decide to move out on their own, falling in love with a place they can't afford. Robin learns of Lily's bad credit rating due to her compulsive shopping for designer brands, and forces Lily to tell Marshall. Despite this, they are able to finally secure their dream apartment, only to discover it's in a bad location and more poorly constructed than they thought (the floor is tilted). Barney is slapped for the third time on Thanksgiving, which Marshall dubs "Slapsgiving."
Ted tells his children he met their mother through a story concerning her yellow umbrella. He finds the umbrella at a club and takes it home after attending a St. Patrick's Day party where his future wife was, although they did not meet. Ted attempts to woo Stella (Sarah Chalke), a dermatologist he sees to remove an embarrassing butterfly tattoo. This culminates in a memorable "two-minute date," which incorporates small talk, dinner, a movie, coffee, two cab rides, and a goodnight kiss, all within two minutes. Robin sleeps with Barney after he comforts her following a break-up with a past Canadian love; Ted is infuriated, and decides to stop being friends with Barney. Meanwhile, an unknown woman begins to sabotage Barney's attempts to hook up. His saboteur is revealed to be Abby (Britney Spears), Stella's receptionist, with a vendetta against him for not calling her after they had sex.
In the season finale, after Ted and Barney get into separate car accidents and end up in the hospital, they renew their friendship. It is revealed Barney has true feelings for Robin, while Ted proposes to Stella in an arcade.