Frasier exists in two Valentine's Day universes, one with a beautiful but clumsy woman named Monica (played by Charlotte Ross) and the other alone.
Tropes:
- Contrived Coincidence: In the timeline where Frasier doesn't have a date, he still ends up at the KACL party, because Roz set up a blind date for him...who turns out to be the most annoying girl he had earlier met at the speed dating event.
- Cute Clumsy Girl: Monica; in the timeline where she meets Frasier, she not only breaks his arm when they meet, she also injures his head while they're at the hospital.
- Desperately Craves Affection: In the timeline where Frasier hooks up with Monica, he goes to extravagant lengths in order to impress her and have as lovely a relationship as the one between Daphne and Niles, such as sending her a dozen roses and dedicating a song to her at a KACL party. This ends up freaking Monica out, and she decides to break up with him.
- Grand Romantic Gesture: Frasier ends up alienating Monica by going completely overboard with these.Monica: Don't get me wrong, Frasier. Everything you did was nice, it was just... too much. You made me feel like a project and not a person.
- Hypocritical Humor: In one timeline, Daphne is happy upon learning that Martin already packed her bags when Niles had secretly prepared a trip to Cancun for them. In the other, when the trip is cancelled due to Niles' allergic reaction, she's upset that Martin went through her unmentionables.
- In Spite of a Nail: Some events are constant across both universes, such as a phone call from Niles asking for help preparing his romantic vacation with Daphne, Frasier and Roz attending a KACL party, and (as seen in The Tag) Martin and Eddie just having a nice time together. Most notably, at the end of the episode in both timelines, the new KACL employee and Monica are in a relationship, and Frasier is in his car, alone, deciding to meet up with a caller to his show that had shown interest in him.
- Instantly Proven Wrong: When Roz asks Frasier if he's feeling insecure and lonely because of his brother's relationship, Frasier says that it's a "tiny" bit bothersome...and then Niles shows up wearing a shirt with his and Daphne's faces in a giant heart printed on it.
- Intimate Healing: In one timeline, Niles and Daphne have to cancel a romantic vacation together because Niles has an allergic reaction to her cooking, since Frasier's arm was too injured by Monica to help carry groceries and warn Daphne when she was preparing it. Their moods lighten up immediately when they realize she'll have to rub lotion all over him to alleviate his ailment.
- Point of Divergence: The diverging point between the two universes is whether Frasier chooses to wear a sweater or a suit before attending a speed dating service. With the suit, which he was currently wearing, he ends up meeting a new KACL coworker he introduces to Roz, and then gets accidentally injured by Monica who offers to take him to the hospital. With the sweater, him stopping to change outfits causes the coworker and Monica to bump into each other, followed by going to the speed dating service and striking out.
- Serenade Your Lover: The final straw for Monica is when Frasier decides to perform "Oh Babe, What Would You Say?" for her during the charity ball. The performance ends with her trying and failing to cover the microphone while saying "We need to talk".
- Shout-Out: The entire episode, including the title, is inspired by the film Sliding Doors.
- Sickeningly Sweethearts: Niles and Daphne, in the timeline where Niles doesn't get sick, are constantly giddy to the point of annoying a dateless Frasier.
- Speed Dating: Frasier has agreed to attend one of these events after getting sick of playing third wheel to Niles and Daphne. The timeline split is caused by his trying to decide what to wear to the event. In one timeline he gets hurt and has to miss the event. In the other he makes it to the event but fails to hit it off with anyone and comes home without a single phone number.
- Split Timelines Plot: The episode keeps switching between parallel timelines depicting the two different outcomes of the decision Frasier makes at the beginning.
- Tempting Fate: The episode begins with Frasier trying to convince Roz that something as simple as a single decision could completely alter his destiny, with the rest of the episode proving this is the case...well, mostly.
