Fear and darkness loom above the lives of the citizens of Ente Isla, as the Demon Lord Satan wages war against humans to rule over the world. However, just as humanity nears defeat, their champion, the hero Emilia, successfully brings Satan's conquest to a standstill. This quandary forces him to retreat to a random world known as Earth, alongside his most loyal general, Alciel.

Vowing to one day return, Satan decides to recover his powers in this new world. Unfortunately, Earth ends up being devoid of invaluable magic. Even so, he remains undeterred from his goal of world domination. To cope with the disadvantage this world brings him, he disguises himself as a human named Sadao Maou. Moreover, he starts to work part-time at a fast-food restaurant, while Alciel finds a way to restore their magic reserves. Soon enough, however, Satan realizes that he wants to conquer both Ente Isla and Earth.

Thus begins Satan's journey of becoming a full-time employee and climbing to the top of the corporate ladder. Yet, Emilia suddenly appears, throwing cold water on his plans once again, declaring that she's going to finish him off for good.