This novel follows the story of Kuroki, who is suddenly summoned to another world by the Demon King Modes. The Demon King's kingdom is on the brink of destruction by a hero, and in a surprising turn of events, the villainous-looking Demon King begs Kuroki for help. Kuroki becomes a dark knight and works to save the Demon King. (Note: The story also features a childhood friend on the enemy side, hence the 'Wife Stealing' tag, although it's a minor element.)