A lonely middle schooler named Yukiya Takato moves to his mother’s sleepy seaside hometown, struggling to fit in. One day he hears agonized moans coming from a secluded sea cave called the “cave gate” and discovers a beautiful young man who is injured and cannot speak. The young man possesses the uncanny ability to transform into various animals and even shrink down to a child’s size. Yukiya names him Nagi, and the two lonely souls form a deep bond that soon blossoms into love. But Nagi hides a horrifying secret: he is a supernatural creature from the sea, possibly a mermaid or a “monster” that local legends warn about. The story weaves between past and present, following Yukiya from his first encounter as a middle schooler to his return at age eighteen, when he finds Nagi again. The coastal town is steeped in dark folklore, and the local community holds hidden knowledge about Nagi’s nature and a mysterious festival involving “mermaid water” that can transform people. As Yukiya becomes more entangled with Nagi, he uncovers a history of violence, imprisonment, and a centuries-old conflict between the creature and the town. The narrative arc swings between tender romance and grotesque horror, exploring themes of obsession, forbidden love, and the inescapable pull of destiny. A classmate named Yoda also becomes a key figure as an adult, adding tension and potential rivalry. The manga balances quiet character-driven moments with shocking twists, building a complex mythology around Nagi’s origins and the town’s secrets.