The story of Haiyore! Nyaruko-san centers around Nyaruko, a formless Cthulhu-deity of chaos (Nyarlathotep) who can take on the shape of a seemingly ordinary silver-haired girl. Mahiro Yasaka is a normal high school boy who is being chased by a fearsome black alien one night, until Nyaruko saves him. She explains that the creatures from H. P. Lovecraft's works are actually races of aliens, and that she has been sent to Earth to protect him from being kidnapped by an alien trafficker. Eventually, Nyaruko and two other Lovecraft-creatures, Cthugha and Hastur, end up being freeloaders at Mahiro's place.