In a world where countless heroes and demon lords coexist, Allen is ranked as the number one hero and carries the title of the Strongest. Despite his prestige, he suffers from relentless orders to subjugate demon lords, constant harassment from the king, and pitifully low wages that barely cover the costs of his own expeditions. He is at his breaking point financially and mentally. When he is sent to defeat a new demon lord, he instead meets a beautiful, enigmatic female demon lord who, rather than fighting, offers him a job with exceptional pay and benefits. Allen hesitates but eventually accepts, switching sides to work for the demon lord.

The demon lord has a hidden agenda: to conquer the world in a fair and humane way, contrasting the corrupt kingdom Allen left behind. Together, they face opposition from the human kingdom, which sends increasingly powerful heroes to eliminate them, and from other demon forces. As Allen settles into his new role, the party grows, recruiting allies such as Sarah, a devoted maid, and Ranpo, a money-obsessed hero. Later, the arrival of Kiskill, the mother of the demon lord and wife of the greatest demon lord Lucifer, unveils deeper secrets about the demon lord’s past. The story follows Allen and his allies as they battle both human and demon adversaries, blending workplace comedy with action and exploring themes of labor exploitation, loyalty, and the pursuit of a better world.