[[caption-width-right:286:[[ThereIsNoKillLikeOverkill Yeah, that oughta do it.]]]]

''Of Unknown Origin'' is a horror/thriller film from 1983. It was directed by George P. Cosmatos[[note]](Who also did ''Film/RamboFirstBloodPartII'', ''Film/{{Cobra}}'' and ''Film/{{Leviathan 1989}}'')[[/note]] and stars Creator/PeterWeller, Jennifer Dale and Shannon Tweed. It is [[TheFilmOfTheBook based on the novel]] ''The Visitor'' by Chauncey G. Parker III.

When Bart Hughes' wife and son leave their house for two weeks, he thinks it will be the perfect time for him to focus on a project that'll be sure get him a promotion at work. He's in for a nasty surprise, as a ''huge'' rat scuttles into his domain and wants to make the place its own.

!!These tropes are of known origin:

* AgonyOfTheFeet: When the rat attacks Bart in the basement, it bites his foot first.
* AnimalNemesis: Bart starts obsessing over the rat so much that he refuses to leave the house.
* BatterUp: As the conflict between Bart and the rat intensifies, he picks up a baseball bat to keep himself safe. Later, he jams some nails and rat trap parts on it.
* CampingACrapper: The rat gives Bart a scare when Bart tries to go relieve himself in the middle of the night and finds it inside his toilet bowl.
* ItCanThink: This rat seems to know exactly what to do to tick Bart off.
* MamaBear: The rat attacks Bart when he finds its litter in his basement. And then he drops them in the sewer.
* NightmareDreams: Bart has a nightmare of the rat attacking his wife when he's sleeping in the bathtub.
* PrettyInMink: Meg wears a blue fox coat in her scenes. As she and their son are away most of the movie, her fur is hence one of the luxuries not destroyed by the end.
* PyrrhicVictory: Bart wins in the end, but his apartment is in pretty bad shape.
** With every thing else destroyed in the apartment, Bart finds one unbroken vase, standing on a bookshelf. With a "Oh, what the hell" expression on his face, he tips it over to smash on the floor.
* RainOfBlood: Bart discovers his recently adopted cat's corpse on top of his fridge when he tries to have a glass of milk and its blood starts dripping on it.
* RodentOfUnusualSize: The rat in this film is bigger than a cat.
* ShowerScene: First appearance of Meg is her having a shower.
* TitleDrop: Appears during Bart's studies about rats, on a microfilm screen.
* YourCheatingHeart: Averted. When Bart's secretary, Lorrie, is visiting him, they have a close moment and he is about to kiss her...but the rat scuttles by and he hurriedly ushers her back outside.