During a winter long ago, near-immortal vampires began plaguing the world. As their population grew at an astounding rate, they promptly threw humanity off its place at the top of the food chain. Desperate to avoid extinction, the surviving humans concentrated in their last safe haven—a small city surrounded by an enormous wall. There, they have waged war against the vampires for many years.
In the midst of a particularly brutal attack by the vampires, a human soldier named Momo encounters Fiine, the queen of their enemies. After seeing her try to save Momo's best friend from a rampaging vampire, Momo follows Fiine back to her house. It is there Momo connects with Fiine over their love for music and desire to escape the fighting.
Pursuing that goal of escape, the two decide to journey toward a supposed paradise where humans and vampires coexist—despite being uncertain of its existence. However, Momo and Fiine are being targeted by both races, who they must first evade if they hope to reach their destination in one piece.