This sort of episode only happens in a show with a setting (i.e. otherwise mundane, fantasy, or even sci-fi) which does not normally feature extraterrestrial beings or creatures. And no, the episode isn't just about Xenomorph Aliens in particular.
Anime & Manga
- Episode 22 of Excel Saga sees the Earth under attack from Puchus (who are revealed to be a race of galactic conquerors), which the show promptly uses to cram the episode choke-full with shout-outs to classic Space Opera anime series like Space Battleship Yamato, Mobile Suit Gundam, and Captain Harlock.
- Axis Powers Hetalia had the movie "Paint it, White" where the nations try to ward off the threat of human extinction from the alien race known as Pict, who seek to transform mankind and drain the color from the world.
- The Tintin story Flight 714 starts out normal enough, when Tintin and his friends are kidnapped by Rastapopulus's henchmen and kept prisoners on a small Indonesian island. But it soon become clear that something weird is going on, and it turns out that aliens have been coming to the island for millennia. And yeah, everybody except for Snowy forgets all about the adventure due to Laser-Guided Amnesia.
- Asterix and the Falling Sky is basically its equivalent in the Astérix franchise. Two kinds of aliens (a Captain Ersatz of Mickey Mouse and his Ersatz Superman bodyguards vs Manga-like insectoids and robots) fight over the right to get the magic potion. And yep, it ends with the good toon-like aliens erasing the villagers' memories of this episode.
Films — Live-Action
- The Indiana Jones series typically revolved around religious and mythological artifacts; but Indiana Jones and the Kingdom of the Crystal Skull involves Indy chasing after the crystalline skulls of Ancient Astronauts.
- The Benson episode "Close Encounters on the Third Hole," wherein the Governor thinks a bright light he saw while golfing was aliens abducting Benson.
- The Miami Vice episode "Missing Hours" — the otherwise normal TV series sees lots of weirdness, including a woman claiming her deceased husband had been abducted by aliens and Crockett and Tubbs later see a UFO.
- The MacGyver (1985) episode "The Visitor" has our hero dealing with mysterious happenings involving apparent flying saucers. At the end, after proving that the flying saucers were a hoax, he sees another, apparently real, UFO and it's implied that one of the people helping him out was actually an alien.
- Baywatch Nights had an episode, "Symbol of Death," where Mitch and Ryan deal with a man who insists an alien invasion has long been underway.
- Baywatch: the episode "Strangers Among Us" involves the arrival of a bunch of UFO-logists to the heroes' beach, thinking that aliens will arrive. A bunch of them almost get killed when they mistake the reflected light of a bunch of fish for an underwater UFO and willingly walk into riptide. And in the final seconds of the episode, the Girl of the Week gets abducted for real.
- Soap: A Story Arc involves Burt being abducted by aliens, who send an alien disguised as Burt down to Earth.
- Quantum Leap: Sam leaps into a man who claims to have seen aliens, and as of the end of the episode it appears it might be true.
- Played with in Murdoch Mysteries: The episode entitled "The Annoying Red Planet" starts with a seemingly impossible crime (a corpse hanging in a tree with no footprints leading up to it), and quickly brings in locals' reports of strange noises and lights, crop circles and strange footprints. Constable Crabtree seems quite convinced that aliens (specifically Martians) have visited, but Murdoch is convinced everything has a mundane explanation. It turns out Murdoch is right, but the truth involves some advanced new technology, and there is a conspiracy to make people think aliens are about the place.
- Buffy the Vampire Slayer
- The episode "Bad Eggs" has the crew fighting a nonparanormal parasite — one of earthly origin, but decidedly alien-like.
- "Listening to Fear" featured, as Giles put it, "a killer snot monster from outer space" as the Monster of the Week, summoned to Earth to kill the still-living victims of the Big Bad.
- The episode "String Theory" involved the investigators discovering a number of deaths that all connected with each other, Six Degrees Of Separation-style. The couple of deaths that started the plot involved some junkies taking home-made drugs with unusual chemicals (which killed them in horrifying convulsions and bleeding from every orifice) that turned their blood avocado green, which made a couple of one-shot characters believe that they were seeing aliens in the flesh.
- The episode "Leaping Lizards" involved a man's plan to force a delusional woman to kill his wife (and a few other people) by means of convincing her that said people were lizard aliens that walked around infiltrating society in preparation for the future invasion, and the only way to stop them was by tearing them to pieces with a sword (dressed like Xena: Warrior Princess). The man was not that sane himself, imagining that Brass had a lizard tongue at one point... or did he?
- Time Trax, a show about a future cop apprehending temporal fugitives in the 20th century (no, this isn't Continuum). For most of the series, there's not a single mention of aliens. Then, out of the blue, SELMA detects a signal she identifies as belonging to a race of Human Aliens called Procardians. Darien is shocked, as Procardians have only made contact with Earth in the 22nd century, and their diplomatic ship is preparing to arrive. He eventually learns that Procardians have secretly visited Earth in the 20th century and determined humanity to be too primitive and violent for First Contact (apparently, they change their minds by the 22nd century). The signal came from a crashed Procardian ship, piloted by a Procardian teenager, who has stolen a ship and returned to find his mate, who was accidentally left behind during the previous visit. After helping the alien find his mate, he sends them to his own time (they have no other way to get home, as his ship self-destructed) to await the arrival of their kind. And then his life-altering experience of meeting genuine aliens is never mentioned again.
- The Bones episode "The X in the File" deals with a UFO-logist finding something alien-looking in a field, which turns out to be the mutilated victim-of-the-week (human). The episode has a number of atypical scares, such as the victim's corpse suddenly sitting up during an MRI scan, causing Bones to scream like a little girl and jump into Booth's arms (it turns out there was a lot of metal in the body, hence the attraction to the powerful magnet in the MRI machine). The end of the episode is especially creepy, as it has Bones and Booth lying on top of a car hood in the middle of a field, looking up at the stars and musing on the possibility of alien life. Suddenly, all the sounds disappear, including the wind, and both suddenly find themselves very uncomfortable. Roll credits.
- Seven Days: While the show states right off the bat that the Time Travel technology is based on reverse-engineered tech from the Roswell crash, aliens themselves don't appear for much of the show. In fact, there are only a few episode that involve aliens directly. Two deal with an alien convict dubbed "Adam", who is the only survivor of the Roswell crash (turns out it was a prison transport). Another one deals with one of the scientists effectively becoming Adam, after Adam's chip is implanted in his spine to allow him to walk again. One episode has a very Species II-like scenario, with an astronaut becoming possessed by alien... stuff. An odd episode involves a gremlin-like creature from another dimension (so not strictly a space alien). Yet another suggests a demonic attempt to start World War III. But most are run-of-the-mill Set Right What Once Went Wrong plots with nothing strange happening. Even the one episode that involves Parker going into space involve nothing alien.
- The Mario & Luigi games, (and by extension the Super Mario Bros. franchise), have one installment Mario & Luigi: Partners in Time where aliens known as Shroobs invade the Mushroom Kingdom.
- Devil Survivor 2 stands out from other Shin Megami Tensei games by making the main antagonists aliens based off stars rather than demons and gods. While demons are still present and important, the aliens are the main plot drivers.
- "Sentinel" is the only episode of Gargoyles which involves aliens. Elisa and her gargoyle friends Goliath, Angela, and Bronx visit Easter Island and encounter Nokkar, a centuries-old space soldier who's the sole surviving occupant of a secret extraterrestrial military base (and it turns out that the famous Moai statues are actually based on his face). For some strange reason, Nokkar is under the delusional belief that the gargoyles are members of an enemy alien race, so he tries to kill them.
- Scooby-Doo usually chases after (fake) ghosts, mummies, werewolves, etc. But in Scooby-Doo and the Alien Invaders, the gimmick is aliens — complicated by the fact that there are not only bad-guy humans pretending to be aliens, but also good-guy actual aliens pretending to be Earthlings.
- In 2007, Cartoon Network had a "CN Invaded" crossover event involving several different series (Foster's Home for Imaginary Friends, Ed, Edd n Eddy, My Gym Partner's a Monkey, Camp Lazlo, and The Grim Adventures of Billy & Mandy) having episodes involving an Alien Invasion.