Fffffoom! Ted, in a tux, throws back Robin's bridal veil. "I love you," he says. "I used to be a dude," she rather blissfully says back to him. Fffffoom! That would be the alternative, then.
Lily's on board with Ted -- you should share everything, like she and Marshall do. Barney can't believe Lily knows everything, but Lily says to try her. Barney offers a quiz. Yes, she knows a donkey ate Marshall's pants. Yes, she knows he had to have nickels removed from his innards after a bachelor party. And yes, she knows he's never been to the Pacific Northwest because of his fear of Sasquatch. Barney is frustrated. Ted assures him that Lily and Marshall also have a weird degree of interest in knowing things about each other, which is demonstrated by a ffffoom! in which Lily breathlessly follows a story about Marshall brushing his teeth as Ted, on the couch, begs silently for it to end.
Barney tells Ted that he knows exactly what Robin's secret is. He's just sure he does. And what's the secret? "Our, friend, Robin, used to do, porn -- wait for it! -- ography," Barney says emphatically. Lily sees the possibility, given Robin's quality fake-orgasm noises. Ted: offended; Lily: "thin walls." You know the drill. Marshall thinks maybe Robin's married and got married at the mall. He points out that you can get married at the Mall of America, which I can verify is true, since I used to live basically across the street from it. I didn't have to get married there for it to contribute to my fear of malls. The fifteen-foot-tall Lego characters were adequate. Anyway, Ted is not believing that Robin is married -- she hates marriage, after all. Marshall points out that maybe a bad marriage made her so bitter. They all realize that they don't know much about Robin's history in Canada, other than that she talks incessantly -- fffoom! fffoom! fffoom! -- about a "friend" back there who got married too young. Marshall is sure that this is a cover for when Robin is talking about herself; he's convinced that there is no friend in Canada, huh-huh-ing that it's not like he really has a "friend" who wet his bed until he was ten. "Use your brain, Ted," Marshall mocks. Barney thinks there's no way it's marriage -- it's porn, people. PORN!
Marshall offers to bet Barney "anything" that the secret is marriage. Barney will bet back that it's porn. He wants to bet $20,000. Marshall doesn't have it. After Lily rules out Marshall's potentially losing her virtue in a bet, Barney suggests a solution: the slap bet. Marshall is all excited, because he participated in slap bets when he was little. Lily doesn't understand. The (now-infamous) idea is that whoever wins gets to slap the other person in the face as hard as possible. Lily thinks it sounds "so immature," until Marshall offers her the post of slap-bet commissioner. This, she loves. Of course, the position entails involves resolving any disputes that may arise and imposing any punishments. Barney cautions her to put "the integrity of slap bet" above everything, because this is so important. "On your tombstone," he tells her, "it will read, 'Lily Aldrin: Caring Wife, Loving Friend, Slap-Bet Commissioner." Damn. That would be a great tombstone. Marshall says that Barney's will say he was slapped so hard by Marshall that he died. Death slap! Damn, Marshall.
Ted returns, asking whether he should just ask Robin if she's married. Lily tells him to drop it, and he agrees, which is how you know we're going to cut to him trying to pry it out of Robin -- which we do. He is using a complex Scrabble-based method of inquiry. Ted "casually" mentions to her that Marshall thinks she has a husband -- that Marshall is hilarious, huh? Huh? She wants to know if he's asking whether she's married. After some back-and-forth about privacy and secrets and the dangers of refusing to leave well enough alone, Robin announces that indeed, she is married. It was a mistake, but she is married. "Oh," Ted says miserably. Turns out he didn't want to know.