Freelance photographer Kouhei Morioka is traveling to a castle in Germany to take photos of paranormal activity for his friend Hiromi Anzai, editor of an occult magazine. Upon entering the castle, he's confronted by a young girl in a white dress and cat ears who calls herself Hazuki. She takes a keen interest in Kouhei and offers him a kiss, but she instead reveals herself to be a vampire, sucks his blood, and turns him into her slave.

Much to Hazuki's dismay, however, Kouhei is unaffected by her bite. Hoping to escape the castle and her possessive butler Vigo, Hazuki instead forces Kouhei to help her. With the help of his powerful exorcist cousin Seiji Midou, the two make it out safely. Finally free, Hazuki flees to Japan in search of her mother. Not long after Kouhei returns home, he discovers Hazuki has nested in his home, where he reluctantly allows her to stay. Meanwhile, other vampires set out to find the missing Hazuki.

Equal parts gothic and adorable, Tsukuyomi: Moonphase is a charming and mystical story where two unlikely allies form a unique bond in an attempt to defy a society of immortals.