Although Robin and Don have only been dating a short while, he surprises her by suggesting she move in with him. The rest of the gang think it might be too soon, so she agrees to let them meet Don, which she had specifically avoided before. They enjoy his company, but it soon becomes clear Don doesn't know that both Ted and Barney had once dated Robin. Don eventually finds out from Barney and Lily about the past relationships (Don had thought Ted to be gay), and, unable to comprehend that Robin continues to hang out with two of her exes, leaves the bar. Don apologizes the next day and offers to invite them over for dinner. Once he and Robin have left, Barney reveals to Ted he still wants Robin. Ted tries to convince him he doesn't, and shows him the "letter"; Ted explains that every time he broke up with someone, he would write a letter to convince himself why he shouldn't get back with the girl in the future. Ted had Barney write such a letter after breaking up with Robin, although it becomes clear after reading it he still misses her. Ted reads his own letter, and realizes he too misses her. The two end up arguing about who should get to be with her, becoming very drunk, and eventually crashing Don's place, much to Robin's embarrassment. The next day, Ted and Barney apologize for their behavior, but Robin reveals she wants to take a break from the rest of the group and has agreed to move in with Don. Barney decides to still pursue Robin. Several days go by with no word from Robin or evidence that she has moved out, but one day Ted checks her room and finds it empty; all that's been left behind is the blue French horn Ted had once stolen for her.
Meanwhile, after spending a weekend sleeping in separate beds at a hotel, Marshall and Lily realize they both enjoy their personal space and get separate beds for their own room; they even plan to get a third bed just for sex. After going to Don's place for dinner, however, Lily begins to fear sleeping apart could ruin their marriage; Don reveals he and his ex-wife slept in separate beds before divorcing. Marshall isn't keen on giving up his own bed, but after having sex, Marshall ends up climbing into Lily's bed with her.