Batman in My Basement

Some character -- a Rebellious Princess, an alien, The Chosen One or the Mad Scientists Beautiful Daughter -- is on the run, homeless, out of options, and needs a place to hide. It becomes the duty of the first available Ordinary High School Student they come across to stash the hero in their basement/garage/attic/bedroom until they Save The World/defeat the Big Bad/find a portal back to their Magical Land.

The rescuer, whether enthusiastic or reluctant, quickly runs into a big problem. Because they are not a victim of Parental Abandonment, they have to keep up a charade of hiding the character from said parents. Family members start to notice that the character's spending a lot of time in his room, acting rather snippy about people going in there, stashing food, and apparently talking to himself. He will make some quick excuses about practicing lines for a school play.

Meanwhile, the hidden character isn't much help, being prone to doing strange things having to do with their origin, or falling in love with their protector's sibling or friend, or being pursued by one of their own.

When the character is eventually found, a quick Secret Identity, Unpronounceable Alias, or Line of Sight Name will be conjured on the spot, along with an unusual job title to explain their strange outfit. If female, the hidden character will frequently end up as a maid.

As you would expect it is also a popular concept in fanfictions. Mostly in the form of the author him or herself finding his/her favorite and beloved character entering the real world from their media of origin (book, tv, comic, etc.) and befriending them.

Hilarity Ensues.

The Trope Namer is the Batman: The Animated Series episode "I've Got Batman In My Basement," which revolved around a group of local children hiding Batman from the Penguin as he recuperated from a heavy dose of poison. The production team does not think too highly of the episode, citing the studio decision to make the Penguin resemble his deformed counterpart in Batman Returns, instead of the comic depiction of a civilized gangster whose nickname comes from his tuxedo and top-hat, as well as the bumbling antics of the villains.

Anime and Manga

 * Rukia initially hides in Ichigo's closet in Bleach and raids his sisters' wardrobes for street clothes. Ichigo was rather surprised when he found out.
 * Once the whole charade becomes a moot point, she continues to live in his closet. Apparently she just likes it in there.
 * Haruko in FLCL ends up sharing a bunk bed with the main character while masquerading as a maid.
 * This could be considered an inversion from the standpoint that Naota's father invited Haruko in, despite Naota's complaints, so one of the critical points to this trope (hiding it from his parents) does not play a part. Nor is he particularly trying to help and care for her.
 * In Rurouni Kenshin, Seta Soujiro hid the fleeing Shishio. When Soujiro's abusive family (he was an illegitimate child) catch on to the missing bandages and food, they corner the boy in an attempt to kill. He snaps.
 * Subverted in Digimon Savers. Masaru tries make a big point to Agumon that he must not, under any circumstances, be discovered by his family...only for both of them to botch it thirty seconds later. Mom took it pretty well, actually.
 * Well, considering, it's not really surprising.
 * Digimon touches this theme a lot when the action is set in the real world, with the Digimon usually acting as soft toys. The parents usually find out the truth eventually. Digimon Adventure plays the trope straightest with Izzy to the extent that his parents become concerned that "one day at camp" changed him so much. They're a little more bewildered when Tentomon ends up botching it too.
 * But basically averted in Digimon Tamers. Terriermon does play the part of a plush toy (and gets subjected to play dates with Henry's baby sister Suzie), but Renamon is rather skilled at not being seen and she goes off on her own half the time. Takato makes an attempt at this trope with Guilmon, but after about a day he realizes it's not going to work so he finds an abandoned shed or something in a nearby park that Guilmon can live in.
 * Pretty much the entire run of Detective Conan, where the main character has de-aged ten years and lives with his She Is Not My Girlfriend classmate until he can change back. Which he does, briefly, on several occasions. Naturally, more and more people are discovering his secret, including a confidant of the Big Bad.
 * Also befalls Deadpan Snarker Ai Haibara, aka Sherry.
 * In Yes! Pretty Cure 5, Coco lives in Nozomi's room at first. After an incident involving Nozomi's mother, however, the girls immediately go find him another place to live where his presence won't be as questionable.
 * In Tramps Like Us, Sumire keeps a homeless young man named Takeshi, or "Momo", in her house as her "pet". To keep her boyfriend (and dog-lover) Hasumi in the dark, she has to borrow her friend's actual pet dog when Hasumi wants to see "Momo", and Takeshi pretends to be her distant cousin whenever Hasumi unexpectedly drops by.
 * Hasumi later has his own Batman when Shiori Fukushima moves in, threatening to tell Sumire about the night they spent together if he kicks her out(nothing happened, but Hasumi doesn't remember that).
 * A good deal of humor in Code Geass (especially in the side stories) stems from Lelouch having to hide C.C. in his room.
 * Sonic X features Chuck, Chris, and their two servants hiding Sonic and his friends from the world, Eggman, and Chris's parents. At least until Eggman's land base is destroyed and the world becomes aware of their presence.
 * In a Hayate the Combat Butler omake, Izumi saves the title character from spending a night out in the cold (instead of Hinagiku in the main storyline), her father quickly discovers him in her room and he has to take on his alterego - Hermionie Ayasaki - to keep from suffering his wrath, and becomes the family maid for the duration. Kotetsu (the family butler and male love interest) falls in love with the alter-ego, just like in the main storyline.
 * In an episode of Cardcaptor Sakura, are unable to return to their alternate forms because . Sakura struggles to address the problem that  are hiding out in her bedroom.
 * Index from To Aru Majutsu no Index.
 * Parodied in Beelzebub, including the excuses denizens from Hell use to insinuate themselves into unsuspecting families.
 * Fractale pulls a somewhat shorter version of this.
 * In the last arc of So Ra No wo To, the crew hides a captured enemy soldier that they found trespassing on their border (for a harmless reason) to avoid messing the already troubled ongoing peace talks.

Comic Books

 * In Ultimate Spider-Man, Peter once had to hide Wolverine.
 * Jean switched their minds in Ultimate Spider-Man issues 66-71 and Peter hid Wolverine in his basement in issues 34-39 of Ultimate X-Men.
 * A graphic novella called Good As Lily is about a teenaged girl who encounters three different versions of herself: as a kid, as a career woman, and as an old lady. She hides all of them in her bedroom as she tries to figure out what to do with them.
 * The New God named Metron once crash landed on Earth and lost his memory. Because people in the DC Universe are apparently extremely jaded the random little kid who saves him doesn't seem more than slightly surprised.
 * Sodam Yat's backstory has him taking care of an alien that has crash-landed on his planet, which just so happens to be very xenophobic, and stigmatizes any form of space travel/exploration.

Fan-Fiction

 * Twilight Sparkle hides Chirp, in her library in the My Little Pony Friendship Is Magic fanfiction Silent Knight. Cue Crowning Moment of Funny when she buries him under a pile of books to hide him from a patron.

Film - Animated

 * In Pixar's Monsters Inc., Sully and Mike are forced to keep a human child in their apartment and later take her to work with them. They end up having an argument over her in full view of their coworkers, and take the above-mentioned play excuse Up to Eleven by spontaneously breaking out in song. Their deception is made even harder by the fact that if the girl screams  the circuits overload and fuses get blown.
 * The play excuse goes beyond eleven in the credits, where they actually put the show on.
 * Inverted in Tangled. While Rapunzel briefly does keep Flynn hidden in a closet while Mother Gothel is around, she doesn't want to keep him a secret. She wants to use the fact that she knocked out and locked up an intruder as proof that she can handle going outside (It Makes Sense in Context). Mother Gothel refuses to listen and never finds out.
 * Planet 51 looks to be an inversion of E.T, with the plot being about a human astronaut landing in a world populated by aliens and hiding out with the help of a friendly resident alien.
 * Aladdin did this with Iago in The Return Of Jafar, although it was because he wanted to break Iago's Heel Face Turn gently to the Sultan and Jasmine.
 * Briefly done in The Iron Giant, when the eponymous Giant comes to see Hogarth in his house. One of Hogarth's attempts to keep up the charade is a Crowning Moment of Funny.
 * Then done again later (with another CMOF) when the Giant ends up staying with Dean... and the Army comes by.

Film - Live-Action

 * ET the Extraterrestrial and ALF had entire families (or at least the younger set) hiding these friendly, glowy and/or cat-eating aliens.
 * The Fat Albert Movie displays this trope strongly.
 * The entire plot of the 1980s classic Hiding Out, starring Two and A Half Men's Jon Cryer as an accountant who has to go on the run from the mob, or some crooked police, or something.
 * In the 2007 Transformers movie, Sam Witwicky is lucky his parents are clueless enough to not notice that what happened to their backyard had to have been done by someone really, really big, rather than being an earthquake.... Then again, they knew he was trying to hide something, they just thought it was the girlfriend that he'd brought up into his room. The trope has been abandoned by the second movie.
 * Utilized in The Transporter where Shu Qi's character, Lai, is introduced as "The new cook" (as opposed to "the maid") to the Police inspector who is surprised at the change in Frank's (Jason Statham's) normally routine, and deceptively mundane, lifestyle.
 * In Goodbye Lenin a young man's mother falls into a coma in East Berlin; when she wakes up, the doctor tells him that even the smallest shock might send her into a coma again. However, while she was in a coma, the Berlin Wall fell, and the Communist East Germany she knew no longer exists. Thus, the reunification of Germany becomes the Batman in his basement, as he goes to increasingly desperate lengths to make her think East Germany is still as she remembers it..
 * In the film Volver a woman who runs a hair salon out of her house has to keep her customers from finding out that her mother, who is supposed to be dead, is living with her. She convinces them that her mother is a Russian lady who speaks no Spanish in order to forestall any awkward questions. Whenever someone who would recognize her visits, the mother hides under the bed.
 * The first Hellboy movie has a scene of the title character Roof Hopping, when suddenly he runs into a little boy who likes to sit up there with the pigeons. They hang out and eat cookies. It's one of the best moments of the movie.
 * In the 1997 Disney film Under Wraps, three kids hide a reanimated mummy in the main character's bedroom. This is where his obsession with horror film memorabilia comes in handy.
 * In the Soviet classic Ivan Vasilievich, Shurik is an inventor whose Time Machine accidentally brings Ivan the Terrible to his apartment. He is forced to hide the tsar from his wife, nosy neighbors, and the police, while he fixes the damaged machine. The twist is the tsar is a dead ringer for one of his neighbors (who is stuck in the past, impersonating the tsar), so naturally everyone assumes it's him.
 * In Gamera The Brave, a boy named Toru hides a baby turtle from his father. Little does he know,
 * In Iron Man 3, Tony crashlands in Tennessee while investigating the Mandarin, and has to rely on help from a precocious 10-year-old named Harley (no, not that one), out of whose garage/workshop/lab he works, and where he leaves his semi-demolished armor.

Literature

 * In Raymond Briggs' The Man, a boy must hide a tiny and very demanding man in his bedroom.
 * There was a horror story in which a young girl discovers her estranged father has been shrunken and hides him in her jewelry box. She treats him like a pet until he is discovered by her mother, at which point the two of them do...something really horrible.
 * The main character's family in Wizard's Holiday and part of Wizards at War is perfectly aware of the alien exchange students living in the basement, but they have to keep their existence secret from everyone else in town.
 * The beginning of Piers Anthony's Virtual Mode series.
 * In Thomas Love Peacock's satirical novel Nightmare Abbey (1818), the hero finds himself having to conceal a beautiful woman he believes to be a political refugee caught up in a web of Illuminati conspiracies . Their endless (thankfully offstage) conversations on Kantian philosophy don't make up for what happens when his girlfriend finds out. With a clear parody of the trope Older Than Radio, the trope itself is probably at least as old.
 * In The Secret Sharer by Joeseph Conrad, a ship's captain hides a stowaway in his cabin, concealing him from the rest of the crew.
 * This is the plot of the novel The Sugar Queen. However, the girl in the closet turns out to be
 * In one of the earliest Mack Bolan The Executioner novels by Don Pendleton, the protagonist is found by a "Marion the Librarian-type" after he has collapsed unconscious from his injuries following one of this raids on a Mafia target. She hides him in her house from both the police and the Mafia while she nurses him back to health and even launders his battle suit.
 * Not in the basement, per se, but in Harry Potter and the Chamber of Secrets, Harry has to keep a very noisy Dobby hidden from his aunt an uncle during a dinner party.
 * In a short story titled Such A Sweet Little Girl, a Creepy Child has a ghost in her room. Subverted in that the girl in no way makes the ghost's presence a secret (and in fact openly brags about it), but no one in her family believes her.
 * In The Wish Giver,
 * The protagonist of Beachwalker spends most of the book hiding an escaped POW in a basement. It doesn't help that said refugee's enemies have control of most of the town.

Live Action TV
"George Michael: I have Pop Pop in the attic.
 * This is the premise of Nickelodeon's sitcom The Journey of Allen Strange, about a brother and sister hiding a stranded teenage alien in their attic.
 * An inversion in a The Outer Limits episode, two robots clone/birth a human after humanity goes extinct, and have to hide him from the other human-hating robots.
 * Inverted in Buffy the Vampire Slayer when Xander had to hide the villain Spike in his basement, to avoid his recapture by the Initiative.
 * Excellently done on Arrested Development. Michael guesses correctly that his son is hiding someone in the attic, but assumes it's his girlfriend rather than his grandfather. This leads to the following exchange when his son tries to confess:

Michael: What? The mere fact that you call making love 'pop-pop' tells me you're not ready!"


 * Subverted in the Farscape episode "I, E.T." Crichton lands on a planet that's never encountered aliens, and is caught by a young boy. He assumes this is going to be an E.T. story, and is aghast when the boy almost immediately calls for his mom. Then the two argue about what to do, with the adult wanting to hide him and the child wanting to turn him in to the military.
 * Power Rangers ZEO: Bulk and Skull hide an amnesiac Goldar & Rito in their basement for a while.
 * In The New Adventures Of Beans Baxter episode "There's No Place Like Omsk," Beans hides a Russian defector, who happens to look exactly like Miss Universe Shawn Weatherly.
 * In the new Doctor Who series, the tenth Doctor hides himself in this manner by storing his timelord memory in a locket and tricking himself into thinking he is a Edwardian-era boarding school teacher so that the Family of Blood will not track him down and kill him in his past. His companion ends up posing as a beleaguered maid, retaining full knowledge of why her "employer" has good reason to act so out of character.

Webcomics

 * In Sluggy Freelance, after accidentally bringing a man-eating alien into our dimension, Torg ends up giving her a job as secretary to his web design business and letting her stay at his place. He doesn't have to put up much of a Masquerade, however, since everyone who might care about the alien in his apartment either already knows, or is gullible enough to mistake her for a lamp/party decoration/really ugly woman.
 * Misfile: Ash can't exactly hide Rumisiel and Vashiel in his room given the length of the comic's run. Instead he tells his father that Rumisiel's his boyfriend and Vashiel his brother who got kicked out of the family home.
 * Used in Coming Up Violet when Dawn crosses the proverbial pound and is forced to stay with the eponymous character and her guardian's family. Violet then deflects attempts by her friends to go to her house while the still traumatized human girl stays there. Eventually they make a disguise for her so that she can move about freely though.

Western Animation

 * The name of the trope comes from an episode of Batman the Animated Series, in which a boy hid an injured Batman in his mother's basement. It doesn't fit most of the rest of the trope, though, as when the kid frankly tells the mother what he's doing, she assumes he's playing with his friends.
 * In American Dad, Stan and his family hide Roger - an alien avoiding government capture - in their attic.
 * Another Episode, AT: The Abusive Terestrial, has Roger, after a short series of circumstances, starts living with another kid temporarily, similar to ET. However, the plot doesn't focus on the kid hidig him, rather on the fact the kid starts abusing Roger and Steve tries to get him to leave the little punk.
 * The Simpsons: in seperate episodes Bart takes in Otto (kicked out of his apartment) and Chester Lampwick, a hobo who invented Itchy & Scratchy. He keeps them both in the basement, but his parents find out fairly quickly.

Real Life

 * Anyone who's tried to take in a stray animal as a kid has been through this.