The Slutty Pumpkin Returns is the third Halloween episode of How I Met Your Mother.
Future Ted reminds his kids about how ten years earlier he met the Slutty Pumpkin, a girl in a provocative pumpkin costume who appeared to be very compatible with Ted, during the rooftop Halloween party. However, he lost the Kit Kat bar that she had written her number on and waited each year in vain for her to appear again.
In the present, Ted sees the slutty pumpkin costume in the window of a shop, where the owner willingly divulges the identity the woman who rented it in 2001. Ted meets Naomi, the girl who wore the costume and has also been searching for Ted; they begin dating. Ted immediately realizes he doesn't love her at all, but can't bring himself to end the relationship so easily after finally finding the slutty pumpkin girl. When Ted decides that he needs to break up with Naomi, he's stunned to see she's wearing the slutty pumpkin costume and says that he loves her. They arrive at the Halloween party on the roof wearing the costumes they wore when they first met. When Naomi makes the Tootsie Roll drink, she confesses that she found their relationship as awkward as Ted did, but also didn't want to end it after waiting so long to meet him and tried to force him to break up with her by acting strangely. They break up amicably.
Meanwhile, Lily's judgment is impaired by the fact that she has "pregnancy brain," and impulsively decides she wants to live in the suburbs now that she and Marshall have inherited her grandparents' house. Though Marshall wants to live in the suburbs, he does not want to take advantage of Lily's state of mind. When they arrange for a real estate broker to assess the house, Lily tries to trick Marshall into giving in when visiting trick-or-treaters arrive at the house. However, when Lily realizes how her pregnancy brain has affected her ability to make decisions, she decides she doesn't want to live in the suburbs and agrees to wait until she can think clearly before she and Marshall decide whether they want to move away from the city.
In the meantime, Robin learns from Barney's father Jerome Whittaker through Facebook that Barney's paternal grandmother was born in Manitoba, thus making Barney one-quarter Canadian. Barney is horrified and Robin goes out of her way to emphasize Barney's Canadian heritage to compensate for all the mockery she has endured from Barney over the years. She agrees to stop if Barney wears a Mountie costume for the Halloween party on the roof, but Barney resists and winds up wearing an Apollo Creed costume complete with flag backdrop and James Brown's Living in America as background music. When he returns home later, his Canadian side manifests and Barney futilely tries to beat him down.