The gang doesn't believe that it's safe for Ted, a single man, to hang out with Zoey, a married woman. Meanwhile, Marshall and Robin decide to spend some time together. At the same time, Barney and Lily have some strife going on. If only Future!Ted can remember what...
This episode provides examples of:
- A Rare Sentence: "Once a mermaid gets pregnant, she becomes a manatee again. ...never thought I'd say that sentence..."
- Anachronic Order: Barney and Lily's subplot, as it happens, actually takes place sometime during the events of "Now We're Even".Future!Ted: Wait! That's it! That's what Barney and Lily were fighting about! The "Mermaid Theory"! But their argument didn't happen at the same time as all this other stuff. I'm literally in the wrong year!
- Breaking the Fourth Wall: Lily and Barney get noticeably bored and impatient as Future!Ted struggles to remember why they were fighting.
- Captain Obvious: "Wait! Barney isn't a woman!"
- Chekhov's Gun: The titular mermaid theory winds up being the key to Future!Ted remembering what went down in Barney and Lily's subplot.
- The sound of ticking that accompanies the "Mermaid Theory"note ...
- Continuity Nod: Indirect: The eventual punchine of the Barney and Lily subplot can best be summarized thusly: "Once again, Future!Ted accidentally talked about an event that was actually one year off."
- Creepy Good: The Captain, while giving the gang the creeps, turns out to be a genuinely Nice Guy who sincerely wants to bond with Ted because he fears he doesn't always connect with his wife's younger friends.
- "Eureka!" Moment: After Marshall explains the "Mermaid Theory"'s exceptionnote , Future!Ted remembers why Barney and Lily were fighting.
- Foreshadowing: After Ted and Zoey say they don't have feelings for each other, the ticking sound of the Mermaid Clock can be heard, foreshadowing that they will soon develop those feelings.
- Flashforward: This turns out to be the case with Lily and Barney's fight, as Future!Ted ultimately remembers that it happened further in the future than the rest of the events in the episode. After remembering this, Ted is shown walking into MacLaren's in a green dress, which is not explained until "Now We're Even".
- The Friends Who Never Hang: Marshall and Robin realize they never spend any time together independent of the group. When they go out alone together it's... awkward.
- Here We Go Again!: Defied: after Barney and Lily's subplot, which suffered from Future!Ted's bad memory causing bizarre stuff to happen, it finally sorted out at the end, we see Ted walk in, wearing a green dress, telling Barney that they were even now. What seems like another case of Future!Ted's bad memory, he immediately clarifies that there is an explanation.
- I Have to Wash My Hair: Ted invites the gang to a boring event, and they exaggerate this trope to emphasize their lack of interest.Lily: Sorry, I'm, uh, I-I don't know, washing my hair.
Marshall: Running the water.
Robin: Holding the towel.
Barney: (Beat) And I'll be at home trying to get over the fact that no one invited me to the big hair-washing party. - Like Brother and Sister: Marshall and Robin's relationship is shown to be completely platonic, with Marshall even trying to ensure this remains the case by avoiding spending time alone with Robin so that she doesn't become attractive to him per Barney's "Mermaid Theory".
- Mind Screw: As a result of Future!Ted not remembering things clearly, a motorcycle spontaneously drives through the bar, Barney calls himself a woman, and Barney manages to make a beer bottle float in front of Lilynote .
- Rage Breaking Point: After Barney supposedly made a beer bottle float in mid-air, Future!Ted gave up trying to remember that story.
- Running Gag: Marshall covering half of the picture he took of The Captain to only show his terrifying eyes, and everyone screaming in terror.
- Series Continuity Error: While Marshall and Robin find it awkward to hang out alone here, they did not have this problem in "Little Minnesota", where they spend several nights having drinks alone together at a Minnesota-themed bar and, later, a Canadian-themed bar. Marshall also claims that he deliberately avoids being alone with Robin to prevent her from becoming a "mermaid" in his eyes, which also contradicts that prior episode. The first issue is possibly explained by Future!Ted misremembering when this happened as well, since he makes this very mistake with Lily and Barney's fight in this episode.
- Sheep in Sheep's Clothing: The Captain, as it turns out. While Ted is convinced that he intends to murder him, and many of the things the Captain says do sound very much like Implied Death Threats, Ted turns out to be wrong. The Captain really does just want to be friends with him.
- Tempting Fate: After he realizes he got Barney and Lily mixed up in telling their story, Future!Ted declares that the story will now make sense. Cue the floating beer bottle.
- Twitchy Eye: Marshall when the stress of not being able to hold a conversation with Robin gets to him. Robin catches on, and she catches the twitch as well.
- Unreliable Narrator: Played for Laughs with Future!Ted struggling to remember the details of Lily and Barney's fight.Future!Ted: Kids, here's one thing I do remember: I was pretty sure I was going to die that night.
- Wham Episode: This episode reveals Lily will get pregnant in the future.
- Wham Sound: After Ted and Zoey confirm that they don't have feelings for each other, the sound of ticking starts up.
