Long ago, during a winter, near-immortal vampires began to plague the world. As their numbers grew at an astonishing rate, they swiftly displaced humanity from its position at the top of the food chain. Desperate to avoid extinction, the surviving humans gathered in their final safe haven—a small city encircled by a massive wall. There, they have been waging war against the vampires for many years. Amidst a particularly savage vampire assault, a human soldier named Momo encounters Fiine, the queen of their foes. After witnessing her attempt to rescue Momo's best friend from a rampaging vampire, Momo follows Fiine back to her home. It is there that Momo bonds with Fiine over their shared love of music and their wish to flee the conflict. Driven by this goal of escape, the two decide to travel toward a rumored paradise where humans and vampires coexist—though they remain unsure if it actually exists. However, both races are targeting Momo and Fiine, whom they must evade if they hope to reach their destination intact.