As Kouhei Morioka embarks on a photography assignment at a German castle to capture supernatural occurrences, he encounters Hazuki, a vampire posing as a young girl with cat ears. Despite her attempt to enslave him through a kiss, Kouhei remains unaffected, much to Hazuki's chagrin. Together, they navigate a daring escape from the castle with the intervention of Kouhei's cousin, Seiji Midou, an adept exorcist. After their successful getaway, Hazuki ventures to Japan in pursuit of her mother, only to find refuge in Kouhei's home where she decides to stay. As Kouhei grapples with this unexpected new housemate, the duo's bond deepens while evading pursuit from other vampires. Tsukuyomi: Moonphase weaves a tale of gothic allure and endearing camaraderie, as two disparate souls forge an unlikely alliance against a backdrop of immortal intrigue.