Ted starts his first day as an architecture professor, standing in the middle of a classroom in which the mother was present, but it turns out not to be the architecture class he is supposed to teach, instead he is in an economics class. Barney and Robin have had a sexual relationship throughout the summer, but are not ready to label their relationship. However, Lily locks them in a room, forcing them to come to terms with their relationship. They date for a few weeks, but 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. Marshall uses his fourth slap on Barney. Ted dates a graduate student named Cindy (Rachel Bilson) and it is revealed that her roommate is his future wife.

Robin gets a new co-anchor on her show Come On, Get Up New York!, named Don. Although Robin initially dislikes him due to his unprofessional manner, the two start dating, and Robin later moves in with him. Robin later get a call from WNKW in Chicago, offering her the anchor job. Robin turns it down, keeping her relationship with Don ahead of her career, but is devastated when Don tells her that he took the same job.

Lily and Marshall are unsure about having a baby, and they decide that they will start trying when they spot the fifth (Barney's) doppelganger. They finally spot Barney's doppelganger, as a brunette cab driver, and decide to start trying to have a baby. It turns out that the cab driver was actually Barney, and he had worn the disguise to try to pick up women. When Marshall finds this 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. A few months later, Lily thinks she has spotted 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. They then tell Marshall that she is ready to have a baby.