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.

Haiyore! Nyaruko-san (這いよれ! ニャル子さん, lit. Crawling Up! Nyarlko-san) is a Japanese light novel series written by Manta Aisora and illustrated by Koin. It was inspired by H. P. Lovecraft's Cthulhu mythos. Soft Bank Creative has published ten volumes under their GA Bunko imprint. A Flash original video animation (OVA) by DLE titled Haiyoru! Nyaruani was published in between October 2009 and March 2010. A Flash anime television series by DLE, Haiyoru! Nyaruani: Remember My Mr. Lovecraft, aired in Japan between December 2010 and February 2011. An anime television series by Xebec aired in Japan between April and June 2012. A second season is currently airing. Two manga adaptations have been produced.