The story begins when high school student Mutou Kazuki is killed one night saving a mysterious girl from a monster, only to wake up and find it was only a dream. However, he soon finds out how real that dream was when a giant serpentine monster attacks him and his sister. Tsumura Tokiko, the girl from his dream, explains that the monster is a homunculus. Kazuki had been attacked and killed by it when rescuing Tokiko, however the girl revived him by placing a kakugane medal in his chest.