Reborn as a demon lord in a fantasy world, Yuki finds himself embroiled in the politics of a human nation, the Kingdom of Alisia. He and the hero, Nell, along with her holy knight comrades, come up with a daring plan to take back the royal capital from the traitorous prince and save the king. Their first step? Find and destroy the source of the Undead.

However, Yuki is forced to part ways with Nell and the holy knights when, during an impromptu feast with the city's residents, he notices a spy. He follows the enemy right into the royal castle, where he decides to launch a solo rescue of the king. He doesn't have much time to chat afterward, though, because a battle maniac suddenly challenges him to a duel...?!