(Ted notices someone in a penguin costume)
(flashback to 4 years ago when Ted was telling Marshall and Lily about meeting the Slutty Pumpkin)
Ted: She spent a year in Antarctica studying penguins. Barney: Every Halloween, I bring a spare costume in case I strike out with the hottest girl at the party. Go on. He's just this guy I've been seeing for a couple of weeks. Just put on the girlfriend costume for the night. Robin: Are you trying to get me to join a cult? What's that left hand?
Plan: A: the couples costume competition; Q: What are Marshall and Lily excited to participate in? A: their usual bar; Q: Where is the couples costume competition? A: their new relationship; Q: What does Robin's refusal to dress in a couple costume with her boyfriend put stress on? A: Ted; Q: Who makes his annual visit to the rooftop Halloween party in search of a girl that he met three years ago? Summary: Marshall and Lily are excited to participate in the couples costume competition at their usual bar. On the other hand, Robin's refusal to dress in a couple costume with her boyfriend puts stress on their new relationship. Meanwhile, Ted makes his annual visit to the rooftop Halloween party in search of a girl that he met three years ago.