Relationship Reset Button

They've been doing the Will They or Won't They? dance for a while, or maybe they've always been Star-Crossed Lovers. Now finally they've actually gotten together. What does this mean? It means that something's about to happen so that for one—and only one—of them, their relationship never happened. Usually that "something" will either be a form of Easy Amnesia, a suspiciously convenient Cosmic Retcon combined with Ripple-Effect-Proof Memory, or some brand of Literal Split Personality.

This can be a useful if Wangst-ridden way of ensuring that Status Quo Is God while still throwing a bone to fans who want to see their favorite couple together. A relationship that neither party remembers might also fulfill those conditions, but is also much more likely to be subjected to nagging doubts of the "Why does it matter that this happened at all?" variety.

If a character knows this is going to happen, she may leave a Note to Self:.

Compare That Didn't Happen, where the characters are only pretending by mutual agreement that they weren't involved. Not to be confused with Relationship Reboot.

Examples of Relationship Reset Button include:

Anime and Manga

The Ruby/Sapphire arc of Pokémon Special ends with Ruby seemingly forgetting his entire relationship development with Sapphire, presumably as an effect of Celebi's warping of time. However, this is played with as the following arc (Emerald) implies at several points that he does remember and is lying to Sapphire, although this plot thread has yet to be clearly resolved. It's vaguely justified as Ruby is 12. How many 12 year olds do you know want to be in a committed romantic relationship?
Ga-Rei gives Easy Amnesia to Kagura after Kensuke rescues her from the Kyubi.
The first season of Sailor Moon ends with everyone losing their memories of everything relating to the Moon Kingdom and their time as heroes, including Darian forgetting his relationship with Serena. Serena remembers everything before anyone else in the next season, leading to her having restart their relationship from scratch.
Yu-Gi-Oh! 5D's: The Dark Signer arc placed quite a bit of focus on the development of the relationship between Jack Atlas and Carly Nagisa, going as far as mutual Love Confessions and attempted Together in Death when Carly dies and becomes a Dark Signer and Jack is forced to kill her. At the end of the season, however, a Deus Ex Machina brings Carly back to life, stripping her of her memories of her time as a Dark Signer in the process. Their relationship is never developed or brought up again in the later arcs—Carly is reduced to Comic Relief on the same level of significance as the other Fan Girls in Jack's newly-acquired harem, and Jack essentially ignores her for the rest of the series, as if he forgot the whole thing, too. No wonder the shippers are mad.

Comic Books

This was part of the premise for Grant Morrison's Superman 2000 proposal: Clark would have to give up his marriage with Lois to a supernatural being in order to save her life, and only he would remember it ever happened.
- Why does that sound familiar...?

Film

50 First Dates: Every single day for the rest of their lives.
The French film A Very Long Engagement is about a woman who tries to find out what happened to her fiancé after he is seemingly killed in World War I. At the end she discovers that he has amnesia and doesn't know who she is but it is implied that she will stay with him because he is still the same person sans memory and she still loves him.
The Butterfly Effect: The time-resetting hero knows that no matter what he does, his girlfriend will always be miserable if she's with him, so he goes back to his childhood and scares her off. Years later, he crosses her in the street, but of course she has no idea who he is.
Eternal Sunshine of the Spotless Mind is arguably all about this, as it's about selective memory alteration to forget people -- which conveniently turns out to mean that the two main characters, both having erased their memories of the other after a bad break-up, meet each other and begin the same process over again.

Literature

In The Fourth Bear by Jasper Fforde, Mary and Ashley go out on a date and have a great deal of fun, even though Ashley's an alien. At the climax of the book, Ashley pulls a Heroic Sacrifice to stop a van-full of nuclear cucumbers from destroying Reading. As an alien, he has his memories backed up, so he gets better; as he was careless about backing himself up, though, he loses the last two weeks of his life, including the date.
In The Graveyard Book by Neil Gaiman, Scarlett is given a choice to forget about Bod and everything that happened, and chooses to forget. Although they were never actually more than friends, she's the first human person that Bod has a relationship with.
In Otherland by Tad Williams, Paul Jonas and Martine Desroubins fall in love while inside the eponymous computer simulation. At the climax, they learn that he's actually an uploaded copy of the "real" Paul Jonas; the book ends with Martine meeting the real Paul Jonas and telling him about his dear friends who he hasn't met yet.

Live Action TV

The Angel episode "I Will Remember You", in which Angel turns human just as Buffy comes to town, so they can spend the day eating peanut butter in bed (and incidentally having lots of sex). It ends up being a disaster, and they need to beg the Powers That Be for a do-over. Angel remembers; Buffy doesn't.
Season 3 of Farscape sees Crichton split into two identical copies, with alternate episodes tracking each copy. One of them gets into a serious relationship with Aeryn, so he's the one that has to die before the two plotlines can converge again. They do work things out in the end, though...
Stargate SG-1:
- The episode "Unending" provides a subtle (and possibly deuterocanonical) example. Teal'c is the only one who remembers the events of the episode; it's not explicit in the script, but the actors have said they tried to play the episode so as to imply a relationship between Teal'c and Sam during it.
- "Window of Opportunity" has an extremely compressed version. O'Neill is stuck in a Groundhog Day Loop and the only people who remember it are him, Teal'c, and the antagonist of the week; during one iteration of the loop, he takes the opportunity to resign his commission for the express purpose of kissing Carter without violating regulations.
- In "Past and Present", Daniel's Girl of the Week turns out to be a previous episode's villain, courtesy of a Fountain of Youth and some Easy Amnesia. When she remembers who she really is at the end of the episode, she decides to undergo the amnesia again rather than revert back to villainy, forgetting her romance with Daniel as a side-effect.
"Unforgettable" from Star Trek: Voyager manages to arrange a relationship where this happens to both parties in quick succession.
Eureka likes to play around with this.
- Future!Carter, who's married to Alison, ends up in the present; ironically, his overfamiliarity with her leads to them not actually getting together. He eventually gets his memories wiped.
- Similarly, the main cast eventually winds up in a timeline where Henry is married to Grace, whom he barely knows. Unusually for this trope, since Henry is the much more major character, we see their subsequent relationship from the perspective of the one who doesn't remember, and don't actually get to see the parts that Grace knows about and Henry doesn't.
- As an effect of the same timeline shift, Jo (who shifted) now finds that Zane (who didn't) never dated her and in fact hates her, while in her home timeline, he'd just proposed, and she'd waffled, when the shift happened.
- "One Small Step" has a subversion which has nothing to do with altered timelines. S.A.R.A.H. is in an accident; after repairing her, Fargo says some of her memory might have been irreparably lost, and there's no way to know what. When her beau Deputy Andy walks in, there's a long awkward pause, and then S.A.R.A.H. says she'd forgotten... that nobody could actually see her smile.

Video Games

In Final Fantasy IX, Freya's love, Sir Fratley, is wounded in battle and forgets who she is. Although she does eventually meet up with him again, he still doesn't know her.

Western Animation

This temporarily happens after the Relationship Upgrade in Kim Possible, where Kim is hit by a memory-erasing ray that takes some time to wear off, and the very last thing she manages to remember is that she and Ron are dating, to Ron's utter frustration.
At the beginning of the second season of Danny Phantom when Desiree wipes everyone's memories and Danny forgets who Sam is.
The Simpsons had an episode like this, only Marge wound up forgetting her entire ten-year marriage to Homer, even after she recovered memories of everything else about the last decade (the kids, etc.)
On American Dad Stan accidentally erases Francine's memory of the last twenty years when he meant to only erase the last twenty hours. The rest of the episode is about him trying to woo her again in the hope that it will restore her memory.