Kouhei Morioka, a freelance photographer, embarks on a journey to a German castle to capture paranormal activities on camera for his friend Hiromi Anzai's occult magazine. At the castle, Kouhei meets a girl named Hazuki, dressed in white and cat ears, who surprises him with a kiss. Hazuki, however, is a vampire who turns Kouhei into her slave by sucking his blood. But, to her shock, Kouhei is immune to her bite. Hazuki then compels Kouhei to help her escape the castle against her possessive butler Vigo. Assisted by his exorcist cousin Seiji Midou, Kouhei and Hazuki successfully flee. In search of her mother, Hazuki travels to Japan but ends up taking refuge in Kouhei's home where she continues to stay, much to his reluctance. Meanwhile, other vampires seek to find Hazuki. Through a blend of gothic and adorable elements, Tsukuyomi: Moonphase, narrates the story of an unusual bond between two allies fighting against a community of immortals.