Hajime lives with the twin sisters of the family of the Komatsu shrine. Inori is of a very spiritual bent and often tries to use her 'powerful' heirloom rosary to protect her friends and family from evil. The other sister, Kanau, is more practical. She often spends time trying to keep Hajime from groping women, which he is always doing by accident.
Amidst strange vampire-like murders in the community, Hajime has had a dream of a hot female demon called Satan that is lusting after his blood. Soon enough, she appears in his waking life, along with her alter ego Lucifer. After an embarrassing situation at school, Hajime and the shrine twins end up taking Satan/Lucifer into their home. Why are these demons attracted to Hajime, and what do they really want?