Ted begins his job as an architectural professor, standing in the middle of a classroom – although the mother was present, it turns out to be an economics class as he's in the wrong lecture hall. Barney and Robin have had a sexual relationship throughout the summer and Lily locks them in a room, forcing them to come to terms with their relationship. After a rough patch they decide to break up. Robin describes it instead as "two friends getting back together." Barney immediately goes back to his old ways, using the playbook to score with women. Throughout the season Barney and Robin show feelings of regret over their break-up.
Ted dates a graduate student named Cindy (Rachel Bilson) and it is revealed her roommate is his future wife. Robin meets Don, her new co-anchor on her 4 AM TV show. Though she initially dislikes him, the two start dating and eventually she moves in with him. At the end of the season they break up when Don takes a job in Chicago — a job which Robin had previously turned down to stay in New York with Don. Marshall uses his fourth slap on Barney, once again at Thanksgiving. Ted buys a house, which needs to be fixed up badly, but is later revealed to be the future home for Ted and his kids.
Lily and Marshall are still unsure about having kids. After watching four doppelgangers of their group (Lesbian Robin, Moustache Marshall, Stripper Lily and Mexican Wrestler Ted) they decide to leave the big decision to the universe's "infinite wisdom" and start trying when they have seen Barney's Doppelganger. In the season finale, Barney disguises himself to have sex with a girl from every country in the world, and Lily and Marshall mistake him for the final doppelganger. When Marshall finds out, he decides not to tell Lily, fearing she will want to wait even longer to have children. Lily eventually finds out and decides to wait. In the season finale, Lily thinks she sees Barney's doppelganger as a hot dog vendor, which causes the group to realize she is seeing what she wants to see, and play along. Eventually Barney agrees having babies is not a stupid idea and Lily and Marshall should go forth. The season ends with Lily asking Marshall to "put a baby in my belly."