The background is in the distant future, a post-apocalyptic world after the destruction brought about by Armageddon. The war between humans (Terran) and vampires (Methuselah) continues, as a struggle between two factions: the Vatican and the New Human Empire.

Yet, amidst the bloodshed and violence, many desire a peaceful co-existence between the two species. However, standing between them and this goal is the Rosenkreuz Orden, a group of vampire extremists who manipulate the two sides and pit them against each other. To combat them, the Vatican special operations group "AX," led by Cardinal Caterina Sforza, must use everything they have - even a vampire who preys on the blood of other vampires.

Adapted from the light novel series of the same name.