Won't Peter be excited? Otto can be just like dear Uncle Ben!
After a hard day of fighting the forces of not-niceness, the hero goes home for some much needed rest. But who should they see having tea with their momnote (Despite the trope name, it can be any loved one and refreshment) but the Big Bad! The sheer, mind-breaking disconnect of the embodiment of strife, hatred, evil and missing-left-socks engaging in polite conversation over tea and snack will throw the hero into a combination of wordless shock and sheer panic. She invited him in.
This is usually a horrifyingly difficult dilemma for the hero, doubly so if they have a Secret Identity. Attacking the villain outright will endanger their loved one and reveal the secret... that is, if the villain isn't secretly training a gun on them as a form of collateral to enforce a civil, if tense, social exchange. What usually happens is the Big Bad gets the first move and delightedly forces the hero to play along, as if they really were having tea with a friend. Expect the hero, villain and Innocent Bystander to have a tense (and potentially hilarious) veiled conversation.
This is not a gender-specific trope, despite what the name implies. It coule be a mother, uncle, daughter, nephew, or stepfather entertaining the villain. The important part is that this is a relative very close to our protagonist.
A few things can happen from here:
If the Big Bad has discovered the hero's secret identity, and with it the location of their loved ones, he will use the drop in to play head games in order to convince the hero to back off for fear he might kidnap his mom or do much worse. Even if he does nothing, it proves beyond a doubt that no one (hero included) is safe.
The Big Bad doesn't know the hero's identity (but may find out once he enters the door), and is a friend of his mom's or even romantically interested. Awkward. If both recognize the other, the usual vibe is "let's wait 'til we're outside".
The Big Bad was waiting for the hero, met his mom, and now wants to kill both.
The hero's mom is an Action Mom herself, knows who the Big Bad is, and all heck is about to break loose.
An absolutely frightening example happened in Bleach. Ichigo comes home after a long day of training only to find the newest big bad who just spent the last few chapters nearly slaughtering his friends drinking tea with Yuzu. It goes downhill from there. Way down hill.
In Fullmetal Alchemist, Ed finds Winry talking with Fuhrer Bradley after his second fight with Scar. This would've been more dramatic had he known Bradley was a Homunculus, but Ling tells him later that day.
Several episodes later this happens again, this time on purpose with Kimblee. It's at this point the boys have to tell Winry that she's in a hostage situation.
In one of the The King of Fighters mangas, Kyo Kusanagi gets really mad when he finds Eiji Kisaragi, a member of Iori's '95 team... having breakfast with his mother Shizuka. (She had caught him in the warehouse and calmly reduced him with words and her naginata.) Unable to just attack Eiji due to Sacred Hospitality, Kyo silently threatened him with death should he ever step foot in the Kusanagi home again.
Funniest thing? Eiji later shows up again. And has dinner with the Kusanagi parents. Shizuka just laughs, Eiji and Saisyu act almost like man children.
Thobari: I can't take this...It's just getting worse and worse...
Hanabusa: Ah, Thobari, you join in too!
Nanoha A's inverts this late in the season. Yagami Hayate's de facto family is spending Christmas eve in the Hospital with her, then four of her friends (two of whom are trying to locate the master of the Wolkenritter and/or keep the BookofDarkness from being completed) drop by unexpectedly with presents. UhOh.
Suzuka, a friend of Nanoha's who introduced her to Hayate, has met at least three of the Wolkenritter (Shamal, Signum and Vita), prior to introducing Nanoha to Hayate. In the first Sound Stage of the season, Arisa and Vita spend some time in the same bath, but don't learn each other's names.
Rurouni Kenshin - Saitou makes himself known to Kenshin by visiting the dojo while Kenshin is out. When Kenshin comes back, Saitou makes it absolutely clear that he could have slaughtered everyone there if he had felt like it. The revelation sends Kenshin into an Heroic BSOD.
Has happened to Spider-man a few times. Doctor Octopus once dated (and almost married) Aunt May. In Ultimate Marvel there is an infinitely creepier version when Peter comes home and finds Norman Osborn sitting on his couch watching TV and waiting for him.
In another creepy example Peter came home one day to find Venom helping his Aunt May with the laundry. Venom was deliberately invoking this trope; Peter had tried to call in help from the Fantastic Four for their upcoming battle, and Venom was letting Peter know that he knew about the attempt and a second one would go very badly.
Later, when Peter revealed his secret to aunt May, she had this exchange with Peter.
Oh Peter... will you ever forgive me for bringing that awful, awful man (Dr. Octopus) into our home? It must have been excruciating for you, knowing what you knew. Here I was betrothed to a criminal psychotic with all those ugly secrets and you knew it all along.
Though they are still civil to each other next time they meet. Ock even saves her life, though it was partly his fault she was in danger in the first place (he was fighting a villain who copied his robo arms) and he left Spidey to die on the grounds that "you're not a civilian".
A Marvel Adventures Spider-Man story opened with this trope when Aunt May took in Dr. Octopus in a prisoner-rehabilitation program. Spider-Man keeps a worried eye on Dr. Octopus, but gets chased off by May after mistaking Ock's household chores as attacks. Later, it turns out that Ock was trying to access a power line beneath the Parker home as part of a robbery scheme.
Fright Night has this happen... immediately after learning that vampires can only enter your home when invited, Charlie comes home to see the vampire in his living room with his mom. Oh Crap.
In the first Spider-Man film, Norman Osborn has Thanksgiving dinner with Peter since he is roommates with Harry. This happens about 5 minutes after they fight in a burning building on the other side of town. Neither knows the other's secret identity, though Norman figures it out after seeing a wound on Peter's arm that matches the one he gave Spiderman.
In Disturbia, Kale comes downstairs, only to find the neighborhood's very own Serial Killer in his kitchen. Turns out Kale's mom invited him, and she gets really mad when Kale freaks out about him being there.
Blue Steel. Jamie Lee Curtis (playing the policewoman protagonist) goes to her parents home to find the killer she's investigating has invited himself there, claiming to be an old friend.
In Derailed, Clive Owen's character has an affair with Jennifer Aniston but they get robbed in their hotel room by a vicious criminal who calls himself "LaRoche". He rapes Aniston and blackmails Owen with the knowledge that he cheated on his wife. Imagine Owen's horror when he gets home from work to find LaRoche, masquerading as a benign French businessman (if you believe in oxymorons), having tea with his wife while flirting with her and (disturbingly) their sixteen year-old daughter. When they leave the room, LaRoche pins Owen against the wall, gripping him by his testicles while threatening him. The wife's reaction after he leaves: "He seemed like a lovely guy."
Susan Cooper's novel The Dark Is Rising. While Will Stanton is enjoying Christmas morning with his family, the Black Rider (a Lord of the Dark) stops by to drop off a ring. He can get through the house's defenses because Mr. Stanton (Will's father) invited him to enter. Will can't force him to leave because using such powerful magic would endanger his family, so he has to be polite to the Black Rider until he leaves.
In Esther Friesner's Elf Defense, protagonist divorce lawyer Sandra Horowitz finds her mother having lunch with elven king Kelerison (whose ex-wife she's representing), who immediately has worked up all of her mom's Jewish Mother guilt against her.
In Ghost Story, Peter, a teenage boy, sees his friend eaten by the servant of Alma Mobley. Terrified, he returns home to find a dinner party in progress, and Alma herself at the table. He's even pushed to sit beside her. Peter's parents make arrangements for Peter to spend some time with Alma, helping her with chores. Peter is paralyzed and mute with horror.
In Hannibal, Hannibal Lecter is being hunted in Italy by a detective, who uses unethical methods. Lecter instantly befriends the detective's wife at a public event, while the detective is obviously horrified, and can't really tell his wife what's going on without seeming jealous. After the wife leaves, Lecter tells him that "I am seriously thinking about having your wife for dinner."
In the second book of The Whitby Witches by Robin Jarvis, the main character Benjamin Laurenson gets home from school only to find the evil warlock having tea and biscuits with his sister. He doesn't take it well.
An early scene in the second Hunger Games book has Katniss returning from a day of hunting to find her mother having tea with President Snow.
Live Action TV
This has happened multiple times on Buffy, possibly to the extent of a Running Gag.
Angelus dropped by Joyce's house in a late-S2 episode to inform her that he was looking for Buffy and yes, he'd had sex with her daughter. Fortunately, Buffy and Willow were a few steps ahead of him, having warned Joyce not to let him in and magically revoked the invitation.
Buffy walks in on Spike having hot chocolate with Joyce in "Lover's Walk". Joyce had gotten the wrong idea due to an earlier Enemy Mine.
Apparently, this happened more than once. At least, in the episode "Forever", Spike said, "I liked the lady! She was decent. Didn't put on airs. She always had a nice cuppa for me."
Glory also lets herself in and threatens Buffy vaguely.
Sylar in Heroes has done this on a couple of times. Once he was having lunch with Matt's wife to try to convince Matt to remove his powers. Another he went to Bennets home posing as Primatech deliveryman there for Bennet, only to have his wife entertain him while waiting.
In Star Trek: The Next Generation, Dr. Moriarty kidnaps Dr. Pulaski. When Picard goes to resolve the situation, he finds the two literally having tea. Moriarty may have an evil scheme, but aside from that he's a Noble Demon.
In A Muppet Family Christmas, the Muppet gang is staying with Fozzie's mom and Fozzie wants to show off his new stand-up routine. He gets the shock of his life when he sees Statler and Waldorf on the couch ready to unload yet another round of heckling, as his mom explains that they're her friends.
In Teen Wolf, Scott's mom gets asked out on a date by Alpha Peter Hale, much to Scott's horror.
In Team Fortress 2's "Meet the Spy", the BLU spy reveals to the team that BLU Scout's mother is the president of the RED Spy's fanclub.
If you borrow money from the Carta in Dragon Age II, they will show up at Hawke's house early in Act 2, literally having tea with Hawke's mom. This is definitely the "implied threat" version. Snarky!Hawke's response? "Mother spends her days making cake. And poisons. Please tell me you didn't try the cake." (Interestingly, this is one of the few points in the game where there's no "diplomatic/nice" dialogue option available.)
In The Order of the Stick, the Monster In The Darkness (who almost seems designed from the trope Minion with an F in Evil) has tea with its "victims" the way a child has "tea" with it's stuffed animals (as in imaginary tea in empty cups, and yes, it has some stuffed animal friends too). It also cheers on those its fellow villains attempt to kill.
All-Star, a student at the Hyperion Academy, came home for the holidays to find the villainous Doctor XX (who didn't recognize him out of costume) sitting at his family's dining room table. Turns out the villainous doctor had gone to college with his mother and they were catching up on old times.
Played with a bit in Dr. Horrible. The Villain Protagonist knows the girl he's hanging out with is dating his archnemesis Captain Hammer, but really doesn't want to run into him.
It's actually a reversal of the trope, in that the "hero" is well aware of the Dr.'s secret identity and is only dating the girl in the first place because he knows he has a crush on her and it will drive him nuts (it really does). It all drives home the point that not only is Captain Hammer not The Cape, he's a total Jerk Ass who goes out of his way to torment a relatively harmless (up until then) villain in his personal time.
Inverted in the aftermath of Afternoon Tea with the Devil's Daughter in the Whateley Universe. Phase and She-Beast, who knew each other in elementary school, get together and talk about books. Only Phase is the child of the powerful, mutant-hating Goodkind family, who are behind the MCO and the Knights of Purity, so She-Beast's friends think Phase is trying to trick her, and on the other hand She-Beast is the daughter of the dreaded supervillain Dr. Diabolik, so Phase's friends in turn think She-Beast is brainwashing her or otherwise tricking her into spilling team secrets. About the only two people not actually ascribing sinister motives to one side or the other are, in fact, the conversationalists themselves.
Played with in The Guild. Bladezz gets framed for having dangerous weapons by a person in another guild, and comes home to find that he was now dating his mother.
Done literally (well, almost) in an episode of 3 Way. Psycho Lesbian Leslie (who, truth be told, is actually more of a Jerkass than a villain) calls up ex-lover Geri to terrorize her. Frightened but angry, Geri demands to know where Leslie is, even wondering for a moment if The Calls Are Coming from Inside the House. Leslie explains that, no, she's at Geri's mother's house, enjoying a cup of tea. To prove this, she holds out the phone and asks for some more tea, and Geri indeed hears her mother gladly agreeing to the request.
In Atomic Betty Maximus often dates Sparky's mom. In a later episode, Sparky's Mom almost married Maximus Sr.
Danny Phantom, "Reign Storm," when Danny comes home and finds his Arch-Enemy playing chess with his dad. In this case, though, Mr. Fenton's friendship with the Arch-Enemy actually precedes the super-rivalry.
Played for laughs in Invader Zim when Dib comes home to find Zim civilly chatting with the former's Dad.
It gets interesting as it is stated in the commentary that Dib is a poorly made clone and it is quite possible that Dib's Dad and Zim are comparing notes on failed biological experiments.
In Spider-Man: The Animated Series, at one point Peter finds Eddie Brock/Venom making small talk with Aunt May / Mary Jane (so he can show Peter he knows his identity/where he lives). Aunt May even comments on how such a polite person Eddie is.
In The Spectacular Spiderman, Brock in this version began as a friend of Peter's, so this still plays out, but in a somewhat different way. When he first transformed, Aunt May was in the hospital at the time, and during a fight, Peter sees him climb toward her hospital window, and it's shown that Brock had sent her flowers- while Brock's gesture was kind of sincere, it has the same effect as this trope usually does. Also, after being depowered the first time, Brock does a Batman Gambit to trick Peter into leading him to where the symbiote is buried (by making Peter think that Brock had managed to access it/had regained his powers). This includes coming to Flash Thompson's birthday, and when he like the other guests leaves a video salutation for Flash, his includes a comment that the others should let Peter know he is looking for him.
In SpongeBob SquarePants, Mr. Krabs discovers that his mother is dating his arch-competitor Plankton and assumes it's all a plot to get to the Krabby Patty formula through her. Plankton, in fact had no idea he was dating Krab's mom, but now that he does...
A non-villainous example in Batman Beyond: An elderly Bruce Wayne drops by Terry McGinnis's apartment to offer him the job of being Batman. They do this while Terry's mom serves coffee, all without her realizing what's actually going on.
A partial example of this occurs in Avatar: The Last Airbender when Aang, Katara, and Sokka learn that Toph is on friendly terms with Iroh. The thing is, Iroh actually is a pretty nice guy and is only vaguely their enemy by virtue of being Zuko's guardian and mentor, so the scene doesn't end up being particularly tense or menacing. He ultimately ends up outright fighting for the heroic side. Doubles as a Funny Background Event since when the entire Gaang sees Iroh afterwards, their first reaction is complete and utter shock, except for Toph, who just waves hi and gives a big smile.
A mild version of this happens to Kyle in South Park when he sees Cartman talking to his mom and asking her questions about Passover.
Bart was aghast to discover that his Aunt Selma was engaged to his archenemy Sideshow Bob, whom she met as part of a prison pen-pal program and had been dating for months after his release. He flat-out told her that the man she planned to marry was a piece of scum.
In Tuff Puppy Dudley's mom once went out with Snaptrap, and after he dumps him, she's with The Chameleon.
In Bret Hart's autobiography, he talks about how as a kid, he would watch Archie "The Stomper" Gouldie wrestle his father on television, including an angle in which Stomper broke Stu's arm, and then threatened to go to the Hart house and piledrive Bret's mom. As Bret had not been smarted up on how wrestling actually works, imagine the look of horror Bret must have had to see Stomper appear at the Hart House, only for him to hug his mother and collect his paycheck.