An ordinary girl Ayva unexpectedly finds herself drawn into an ancient war waged by a mysterious order against the forces of evil. Waking up in a morgue, she discovers incredible abilities within herself. The search for answers leads Ayva to the Order of the Cross-shaped Sword, a secret society of warrior nuns sworn to defend the world from evil. Now she must find a balance between her new duties as humanity's savior and the life of an ordinary teenager.