High school student Tomose Akira survives a plane crash during a school trip, only to awaken in an unfamiliar fantasy world. Alongside his classmates, he discovers that this realm is ruled by vampires descended from noble bloodlines. These vampires protect and govern human settlements, while humans in turn provide their blood as sustenance for the vampires to survive. Akira also learns a cruel truth: anyone who arrives from outside this world quickly falls ill and dies without immunity to the local diseases. He faces an impossible choice: die as a human or accept vampire blood and live as a vampire.

As Akira struggles with this decision, he reunites with a girl he never thought he would see again: Aya, a blind classmate who died in the same plane crash. She has survived by becoming a vampire herself. Determined not to die, Akira chooses to give up his humanity and become a vampire, aiming to live a long, ugly, but persistent existence in this harsh world.

Akira is taken under the wing of Liv, a powerful and charismatic female vampire who also originated from another world and has lived for roughly two hundred years. Liv belongs to the Battle Axe of Valis bloodline, a combat-focused vampire lineage. She rescues Akira when he is nearly killed by a monster and offers him the chance to become her partner. However, when Akira drinks the blood intended to turn him, he discovers it was swapped. Instead of receiving Liv's bloodline, he consumes the blood of a cursed "Dark Bloodline," a lineage Liv despises due to a painful past relationship with a man from that line. Despite this setback, Akira resolves to master his unwanted powers and prove himself worthy of fighting alongside Liv.

The story follows Akira's struggle to control the dark bloodline's abilities while grappling with deep guilt over the death of Aya, whom he blames himself for failing to save. His first mission alongside Liv ends in catastrophe when he loses control, causing her to lose an arm. Overwhelmed with remorse but refusing to give up, Akira vows to grow strong enough to protect Liv instead of being a burden. The narrative weaves together survival, regret, redemption, and the will to live in the face of despair. Later arcs introduce other classmates who have also survived in the new world, as well as new allies and enemies tied to Liv's past, including a hostile vampire named Cheru who challenges the group during a mission involving a girl named Eva who reminds Akira of someone he once cherished.