Every night, Elva emerges into the dark waters brandishing a sword, defending the island against the surging creatures. As a kannagi—a warrior priest marked by white hair, eternal youth, and tainted limbs signaling an inevitable decline—Elva is both revered and feared by the locals. However, when a boy named Alto crosses paths with him, captivated by Elva's resilience and isolated spirit, a bond forms. Swearing to break Elva free from his ominous destiny, Alto dedicates himself to his kannagi. Over eight years, Alto matures into a resolute and devoted young man, seemingly bringing a glimmer of hope to Elva's afflicted existence. Despite Elva's acceptance of his dark fate, Alto's companionship appears to counter the corruption of the black sea, offering a chance at redemption.