Reborn as a demon lord in a fantasy world, Yuki's hard at work building the castle of his dreams. A mighty demon lord needs an equally mighty abode, after all! As he expends colossal amounts of brain power, magic, and effort to bring his perfect castle to life, an intruder stumbles into his super chill, super strong dungeon—one who just so happens to be a crybaby tomboy hero who's been ordered to defeat him! Hijinks ensue as Yuki deals with her in his own unique way, and after they clear the air, he and Lefi tag along when she heads for Alfiro. Though he's intent on playing the tourist, Yuki has another reason for his impromptu visit: to find out who's behind the attacks on his dungeon. A surprise reunion and dastardly political machinations await the antihero demon lord!