The kingdom of Mikado is a peaceful yet divided place, with a strict caste system separating its citizens. The samurai class is tasked with keeping order and protecting the people from demonic foes, and one young Luxuror-born named Jonathan has always dreamed of becoming one of them. He trains alongside Asuma, an orphan from the Casualry class who is fueled by a desire for revenge after his parents were killed by demons.
However, a tragic incident during their training creates a rift between the two. Over the next ten years, their differences widen until they reunite at the samurai apprentice exam. After passing the exam, the boys learn that forming a personal pact with a demon is crucial in defeating attackers. Jonathan teams up with a demon named Al, while Asuma remains committed to his belief of not making deals with the enemy. Meanwhile, a new threat materializes in the form of forbidden literature that transforms its readers into demonic creatures.
As the war between good and evil rages on, whose convictions will remain steadfast?