When an ordinary high school student named Mahiro Yasaka is ambushed by a dangerous monster, his world turns upside down. Enter Nyaruko, a silver-haired girl who claims to be the shape-shifting deity Nyarlathotep from H. P. Lovecraft's Cthulhu Mythos and a defender from outer space. After saving Mahiro from the alien threat, Nyaruko's affection for him leads to an unexpected cohabitation. Joined by Cthuko, an ardent fan of Nyaruko, and Hasuta, a boy with a deceptive appearance, the quirky trio shields Mahiro from otherworldly perils while adding chaos to his daily life.