This is a Parent with New Paramour situation in which the paramour is an obvious Devil in Plain Sight. Throughout this storyline, the parent will be firmly holding onto the Idiot Ball while their kid(s) take on the role of the Only Sane Man. This sort of plot can also be done outside the family structure, with concerned friends filling in for the kid. The use of the Idiot Ball may or may not be justified if the new paramour has supernatural powers of some sort.
The horror will probably begin with the parent introducing the paramour to the kid(s) and providing a frightful deadline for their marriage ("This is Mr. Evil and we'll be getting married next week! Isn't that wonderful, kids?"). The paramour may be a character previously established as a Card-Carrying Villain, in which case the encounter will probably go somewhat like this:
Kid: To who, Mom?
Satan McEvil: You can call me "daddy."
[Kid looks horrified. Cut to commercial break.]
Regardless, the paramour will almost certainly behave in an obnoxious I-got-your-Mommy/Daddy-wrapped-around-my-little-finger manner throughout the scene just to make it absolutely clear from the start that they are bad. The parent, of course, will completely miss this. Once the parent is offscreen, expect the paramour to tell the kid(s) exactly what's what, usually consisting of the revelation that they are a Child Hater and plan to send the kid(s) Off to Boarding School. The introduction of the villain is often a Villain Over for Dinner situation.
The parent will sometimes say, "I would never marry someone if you were dead set against it." And then do just that.
Of course, sometimes, the villain really does not have an evil plan regarding the marriage; they just fell for an incredibly inconvenient individual. Or they do have an evil plan, but only because they really do want to stay with their beloved forever.
Naturally, it will be up to the kid(s) to get rid of the paramour since Adults Are Useless. The solution may involve a Kids Play Matchmaker plot, where the kid(s) try to find a different, better partner for their parent.
Often the plot involves everyone else (other than the good partner and evil suitor) holding the Idiot Ball vis-a-vis the protagonist, where the protagonist is trying to alert others to the danger, but they assume the protagonist is just being petty, jealous, or crazy (or some combination thereof), and start disliking the protagonist based on that.
- In Akuma de Sourou (and the Taiwanese Live-Action Adaptation Devil Beside You) the announcement that the heroine's mother is going to marry again isn't that traumatic... until some days later, when the mother reveals that her fiancé is her daughter's school director, and that his son (a boy with whom the heroine has had some nasty clashes recently) is coming to dinner right now.
- In Heat Guy J, Monica's mother Christina has met a man while out partying late one night, and plans to leave Judoh with him. Monica is suspicious of him, and refuses to go with them. Christina leaves with her new boyfriend anyway, thinking Monica would join them later and they'd be a family. It turns out Christina's new beau has a bunch of women he tricked onto getting on the boat so he could dump them off or sell them as sex slaves and sell their I.D. s to people hoping to enter Judoh illegally. Daisuke and J save the day, of course.
- A Silver Age Spider-Man plot of yore centered on the nuptials of the widow May Parker and one Dr. Otto Octavius.
- Doc Ock was actually marrying May because she was the heir to a private nuclear reactor. He didn't even realize that Spider-Man was her nephew until after he unmasked in Civil War. Brilliantly, he then went into a rant about how stupid he was not to figure it out and how he should have kept up the marriage facade for far longer. It is however implied that he actually likes her.
- An early continuity nod had Aunt May, shortly after learning Peter's secret identity, witness a fight between Spidey and Ock, and finally realise who Octavius was. However, this was a Retcon imposed by the Rule of Funny; in the Silver-Age stories Aunt May, despite her Cloud Cuckoo Lander tendencies, was quite aware who Otto was. One could say it was a case of the Stockholm Syndrome at work as she first developed feelings for him when he charmed the socks off her while holding her and Betty Brant hostage. She found his polished manners much better than those of "that awful Spider-Man".
- For added Brain Bleach fodder, Dan Slott had Peter Parker exchange bodies with Doctor Octopus and forced him to relive Otto engaging in a bit of pre-marital sex in Amazing Spider-Man #699.
- Played with in Ghost World. Enid is talking about her ex-stepmothers, and hates one in particular. Guess who Mr. Coleslaw's remarrying? Subverted in that the woman looks quite normal (it's been a few years) and, if I recall correctly, is actually supportive of Enid and is sympathetic when Enid fails to get into art school.
- A version appeared in Disney's Aladdin, except that Jasmine's father was announcing who she was marrying. And he was hypnotized.
- The Iron Giant has "Guess Who's Renting Our Spare Room", with the paranoid and crazy government agent setting up shop to spy on Hogarth while the mother is rather oblivious.
- Her obliviousness can be justified by her very demanding work schedule, which she has to keep up to care for Hogarth and herself. The agent even brings this up at one point, telling Hogarth that his mother is under a lot of strain as a single mother and that it would be not hard to make it look like she was too neglectful to keep custody of him.
- Monsters University Has Don. He decides to marry Sheri at the end of the film. He tells her son not to worry and that they're Fraternity Brothers first. As if realizing the Unfortunate Implications, he asks her son to think of him as a Frat Brother who is marrying his mother, which he quickly realizes sounds even worse.
- Obviously, this is used in every version of The Parent Trap.
- The "comedy" Mr Woodcock with the villain being a former Drill Sergeant Nasty gym teacher.
- Subverted: it turns out that, rather than Mr. Woodcock being the bad guy, he's just kind of a Jerk Ass, while the son is actually an asshole who has a unforgiving grudge toward his teacher. After realizing this, he forgives his teacher for 'molding him into what he is today'
- Back to the Future Part II with the marriage of Lorraine Baines and Biff Tannen in 1985-A. It begins with them already married, but Marty has the typical reaction since he possesses Ripple Effect-Proof Memory - and since he isn't the Marty from this timeline, who is apparently in Switzerland.
- Problem Child 2 had the titular kid's dad almost get married to a child-hating millionairess. In this case his blindness is arguably justified. Junior had spent the earlier part of the film driving away every other date he had just because he didn't want to share his dad with anyone. Naturally, his dad is skeptical when he tries to tell him "No, this one really IS evil!" Somewhat qualifies as a Family-Unfriendly Aesop: "When your kid drives away your other dates, you can't trust him anymore when the real evil woman comes along."
- Three Men and a Little Lady but with useful adults (the three men) who foil the antagonist's plan.
- The Night of the Hunter, in which the extremely wicked stepfather has apparently Brainwashed the mother. And then he kills her, leaving him all alone with the kids...
- Cpt. Vidal from Pan's Labyrinth is not the ideal stepdad, either. He shoots her! She had sedated him, but he didn't know that. And her nanny had repeatedly stabbed and joker-fied him. He was about to torture her though, so... yeah.
- The Disney Channel movie Mom's Got a Date with a Vampire is Exactly What It Says on the Tin.
- Happened in Nanny McPhee: The great-aunt was willing to cough up money only if the dad married, and as the dad was almost broke, he was desperate... but he didn't quite catch the level of hate between his children and his fiancée. All the more because the kids are ridiculously ill-behaved, and so he didn't quite catch the deliberately malicious undertones to their antics.
- In a bit of a subversion, the father didn't like his fiancée at all either. The only reason he was marrying her was that the alternative was the family going broke and his children being sent to work houses, or who knows where in the case of the younger ones. Subverted even more at the end, when he marries Evangeline, whom he and the children all love.
- In Disturbia, the protagonist's mother steps into the kitchen with the next-door neighbor... whom the protagonist suspects is a serial killer. He is.
- Hank Azaria, in Run Fat Boy Run, actually starts out as The Ace but ends up invoking Derailing Love Interests when it turns out he expects the mom and kid to move to America with him (without ever having mentioned or discussed it before) and is a bad sport in the climactic race just as Simon Pegg's character gets his act together at the end.
- Addams Family Values. Uncle Fester is preparing to marry a seemingly sweet woman named Debbie Jellinsky, who is actually a Black Widow, a killer who marries rich men, then kills them for their money. Only Wednesday and Pugsley are wise to her true intentions, and Debbie deals with them by sending them away to summer camp.
- Sling Blade features this trope prominently as a mentally challenged protagonist befriends a boy and discovers that the boy's mother is about to marry a man who will definitely destroy their lives. He even says so to the boy's face, though not in so many words.
- The Ingmar Bergman film Fanny and Alexander uses this trope, though the mom fairly quickly realizes she's married a monster, and the kids don't free themselves.
- The Sound of Music has Captain Von Trapp bring Baroness Schraeder to his home to meet his children with every intention of asking her to marry him. The Baroness isn't actually all that bad, but the trope holds true from the kids' point of view since she very clearly is only interested in marrying the Captain and his money, not his seven children, whom she'd just as soon pack off to boarding school. Her few interactions with the kids are highly awkward, and not surprisingly, they prefer Maria.
- The mother in All I Want for Christmas does this, making her daughter think her wish to Santa backfired.
- Vince Vaughn is the wicked stepfather in Domestic Disturbance (2001).
- In Stepmonster, Dad's going to marry the eponymous monster (in disguise as a pretty woman). Subverted at the very end, when Dad himself appears with the monster's Weaksauce Weakness (that the children discovered during their Only Sane Man time) and deals with the monster.
- In David Copperfield, David's mother marries the nasty Mr. Murdstone.
- Any adaptation of a fairy tale with a Wicked Stepmother is likely to use this. The Wicked Stepmother, however, is seldom a Child Hater; she loves and jealously protects her own children, if she has any (which is half the problem).
- Some fairy tales start off with the stepmother being kind to the stepchild as well, only for things to go south when said stepchild starts to turn out prettier and more desirable than their biological children (or, in Snow White's case, than the stepmother herself).
- In the novel Prosperos Children, the main character finds out that her dad is marrying an evil witch.
- Happens in Ella Enchanted, when Ella finds out that her father intends to marry the odious Dame Olga for money. She's perfectly lovely to Ella, until she finds out that Ella and her father are broke. Then it follows Cinderella straight through.
- A variation is done in A Series of Unfortunate Events: The Bad Beginning. In this case, when Olaf says "Guess who I'm marrying", it's a lot more squick-y for Violet.
- The backstory of A Brother's Price has a variation where it is the siblings, not the parents, who hold the idiot ball with regards to an intended husband.
- A version with "adult" children occurred in Ugly Betty when Bradford announced he was marrying Wilhelmina.
- Buffy the Vampire Slayer:
- Joyce Summers nearly married an evil robot in "Ted". However, Ted was actually pretty good at seeming nice, so it makes sense that he had everyone fooled. Except for Buffy, because of her tendency to take an automatic dislike to villains before she even gets evidence, combined with him threatening her when her mother wasn't around. Of course, the fact that he was surreptitiously drugging everybody else didn't hurt...
- She also went on a date with Dracula. It apparently went well enough for him to be invited back to her place.
- Uther Pendragon married a troll in Merlin. Disguised as an old sweetheart and enchanting him with magic, but still a troll.
- A heroic variant happens in the Star Trek: The Next Generation episode "Parallels" when Worf realizes he's married to Counselor Troi in one alternate universe.
- This is a big part of the setup for the plot of Hamlet. However, Hamlet did not know his uncle was evil until after the marriage; he just thought Uncle Claudius was an incestuous jerk for marrying his mother so soon after his father died.
- Selma's marriage to Sideshow Bob in The Simpsons.
Bart: That man is scum!Selma: Then call me Mrs. Scum.
- In an episode of The Powerpuff Girls, supervillainess Sedusa sidles her way into the Professor's life in the guise of "Ima Goodlady". note Needless to say, once again, the day is saved thanks to... The Powerpuff Girls! The episode plays with the trope - the girls are perfectly happy with the Professor dating "Ima Goodlady" and even give him dating tips and encourage things as best they can. It's not until they notice that her groundings on them coincide with the nights that Sedusa robs the Mayor that they start to get really suspicious.
- Chef almost married a succubus in an episode of South Park.
- One of Winx Club season 3's subplots was this, involving Stella's dad. They even went the "You can call me mom" route (sans ad break). Amusingly, Stella's reaction differs between the two versions. Original: "Why didn't you tell me about this before?" "Because I wanted it to be a surprise." 4Kids: "Congratulations." "You don't seem too happy..."
- Static Shock:
- A non-parental example that's more like Guess Whom I'm Dating had Virgil feel a bit like this when he found out his sister was dating a member of his Rogues Gallery: Rubber Band Man. Subverted in that she knew very well who she was seeing and they really did care about each other. Of course, the lasting power of the relationship implies that marriage will result sooner or later. It does help that Rubber Band Man is one of the few Bang Babies to give up his criminal ways. He and Static even team up a few times.
- Another example occurs in an episode where Virgil finds out that his father happens to be dating the exact same policewoman that he had a run-in with earlier. She isn't bad, but she's initially suspicious of him because he ran away in a panic when she asked to check his backpack (his Static Shock costume was inside).
- The main character's grandfather, Puma Loco, in El Tigre announces his plans to marry the main villain in one episode. Since they're both villainous, it was for their own nefarious intentions, but they eventually fell in love. Manny took a while to accept it and by the time he did, he accidentally forced them apart.
- Done in the My Little Pony: Friendship is Magic episode "A Canterlot Wedding" with Twilight's reaction to her brother's engagement, though it's only after she's been around the bride for a bit that she suspects her of being evil - she remembered Cadence from her foalhood and immediately approved (though still resenting that Shining Armor hadn't told her before sending the invitations out). It then turns out that Cadence has been replaced by a changeling impostor called Queen Chrysalis, as part of a plan to invade Canterlot.
- In King of the Hill, this plot is run with Boomhauer, his brother Patch, and his ex-girlfriend, now Patch's fiancée. Boomhauer knows his brother is a philanderer, catching him making eyes at Luanne and hiring strippers for his own bachelor party, but everyone else thinks Patch has reformed, and that Boomhauer is just jealous that his brother has his ex-girlfriend. Eventually the truth is revealed.
- The SpongeBob SquarePants episode "Enemy-In-Law" has Plankton dating Mr. Krabs' mother. Mr. Krabs of course thinks this is another plan to steal the Krabby Patty secret formula (which his mother knows) only for Plankton to reveal that he had no idea that she knew the formula. But when Plankton asks Mrs. Krabs to marry him, she refuses, because she's already dating Plankton's giant robot.
- One episode of DuckTales (1987) has Uncle Scrooge preparing to marry a horrible Gold Digger who wants to pack the nephews off to boarding school and basically lay claim to the entire family fortune. Of course the boys have their work cut out for them in persuading Uncle Scrooge what's really happening, so they resort to an unconventional strategy. They get word to Glittering Goldie, Uncle Scrooge's long-ago sweetheart, that he's getting married... and she turns up with a shotgun to run off her competition!
- An episode of Hey Arnold! had a variation, where it wasn't anyone's parents involved with someone who's not all they seem to be, but Helga's older sister, who is in college. She is engaged to a man she's only known for three weeks, who only wants to marry her for her father's beeper business. Helga finds out the fiancé's dirty secret, and plans to keep it because she resents Olga... but then she starts to feel guilty. She convinces the fiancé to leave on the day of the wedding, and writes a fake "Dear Jane" letter for Olga.