Fear and darkness hang over the citizens of Ente Isla as the Demon Lord Satan wages war on humans to rule the world. However, just as humanity approaches defeat, their champion, the hero Emilia, successfully halts Satan's conquest. This stalemate forces him to retreat to a random world known as Earth, alongside his most loyal general, Alciel. Vowing to return one day, Satan decides to recover his powers in this new world. Unfortunately, Earth turns out to be devoid of invaluable magic. Even so, he remains undeterred from his goal of world domination. To cope with the disadvantage this world imposes, he disguises himself as a human named Sadao Maou. Moreover, he starts working 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 the corporate ladder. Yet, Emilia suddenly appears, throwing cold water on his plans once again, declaring that she is going to finish him off for good.