The episode begins and Ted misses a chance to deliver the best opening line to a woman he saw on the subway. He tells Marshall and Lily about his plans to track the woman down, but they put a stop to Ted's attempt to force destiny. They tell him that when Lily came to Marshall's dorm room in college because her stereo was broken that was destiny. Soon enough, the subway woman finds Ted at his school. Turns out, she tracked him down after putting together some clues and possibly set off a fire alarm to find him at his teaching job. Ted is charmed by Jeanette finding him. Robin in the meantime is suddenly bothered by the casual use of the word "stalker," but doesn't want to say why.
Robin finally admits she was a stalker. Back in Canada, she liked a guy who didn't like her and she "got a little obsessed". She admits that eventually "there may have been a teensy-weensy, 50-meter restraining order". "Fifty meters. That's, like, four years" Barney comments. Robin gets defensive and says there's a "fine line" between stalker and charming. She refuses to say who the guy was, and Barney agrees to drop it. But he then breaks into her place and finds her diaries. He finds notes to a guy, and they all end with, "P.S. I Love You". He as a result heads to Canada. Ted asks Jeanette if she pulled the fire alarm.
She did, but it charms him. At Ted's school, Marshall sees there are actually no fire alarms, so Jeanette must have started a fire. In Vancouver, Barney interviews a guy that Robin used to date at a Tim Horton’s. This guy leads to another guy, who tells him Robin was "way more into this other guy". This leads to Simon Trembly, who says he was serious enough with Robin to pop the question. So, he got down on one goalie pad and asked if they could keep dating but he gets to stick it to other babes. She said no. Simon says the story of who Robin stalked goes back to the 1996 Grey Cup. Ted is at Jeanette's place and finally asks her if she started a fire.
She admits that she dropped a match into a garbage can, just because she "couldn't stand the idea" of not meeting him. Barney shows up at the apartment and announces that he has the answer to the question of who Robin was obsessed with. "It's Robin Sparkles 4, y'all" he shouts, showing off a VHS tape. They start the tape and it's the "Underneath the Tunes" episode that tells about how "Canada's sweetheart had a dark side". Paul Shaffer, Geddy Lee and Jason Priestley are among the Canadian stars who lament how Robin threw her life away on that Grey Cup Sunday.
Hockey star Luc Robitaille even admits that before a big game, he'd listen to "Sandcastles in the Sand" to "get a good cry on". Robin's next song, "P.S. I Love You," was too dark to be released by the record label. It's an ode to the man she's stalking. In 1996, she'd booked the halftime show at the Grey Cup. That's when she announced that she "was Robin Sparkles". She announces, "My new name is Robin Daggers" "That day is now known in history as the day grunge was born," Alex Trebek says. Marshall gives Trebek a pass because he's not a music guy. Steven Page of the Barenaked Ladies says every Canadian can tell you which Tim Horton’s they were in and which doughnut they were eating.
The list of names of people who Robin could have been obsessed with ran rampant, including Gordon Lightfoot, Joshua Jackson, Corey Hard and Wayne Gretzky. Lee, Page, Priestley, Robitaille, Trebek and even Dave Coulier all say "it wasn't me”. But one name came up more than any other; Alan Thicke. Barney meets him face to face and, tossing the donuts from Thicke's hands, attacks him. A beaten Barney comes back to the apartment and Robin asks what happened to him. He tells her they watched her "Underneath the Tunes" episode and he was fighting with Alan Thicke. In a flashback, Barney is shown telling Thicke that the song was written about him, but Thicke denies it, adding that he thought it was about Coulier. "It wasn't me" Coulier says again.
Lily laughs because Alan Thicke kicked his ass. Robin realizes that Barney is "obsessed". He denies this, saying all he did was break into her apartment, fly to Canada and interrogate her boyfriends". He admits it and Robin finally reveals who she was obsessed with: P.S. I Love You was about "P.S. Paul Shaffer". Lily in the meantime can't stand it any longer, and admits that she broke her own stereo in college and then went knocking on doors to "create a little destiny". She tried several before she found Marshall's, and the rest is history. "I just couldn't stand the idea of not meeting you," she tells him. Marshall is flattered and sees where Ted is coming from.
Back at Jeanette's place, Ted tells Jeanette that it really was destiny because they were reading the same book in the subway. He picks up her copy of the book and notices the receipt still inside. She bought it at the same bookstore, 10 minutes after he did. She then admits she's been following him since he was on the cover of New York magazine, about a year and a half earlier. Old Ted tells the kids that before a man meets the woman he'll marry, "he'll make one, final, horrible mistake for me, that was Jeanette", and they kiss. The episode ends at this point.