In the world of Ente Isla, the Demon Lord Satan has attempted to conquer humanity using an enormous army of demons. But when he almost succeeds, mankind's champion, Hero Emilia, thwarts his plans, forcing him to retreat into a portal that leads to a world unbeknownst to him—Earth.

Along with his faithful servant Alsiel, Satan stumbles upon the modern-day city of Tokyo without any knowledge of the world itself. Completely out of their depth, they gain some information from a police officer, but Satan still has one more problem—money. In order to survive with his greatly diminished powers, he guises himself as a human named Sadao Maou, lives in an apartment, and becomes a part-timer at the fast food restaurant MgRonald's. Determined not only to conquer Ente Isla but also Earth, Sadao strives to become a full-time employee in his job, as he believes that it would be a step forward in his conquests.

Things begin looking up for the once all-powerful demon as he ventures through his everyday life—his job, his home, and his relationships. However, a major setback pops up when Emilia, the hero who drove him out of Ente Isla, suddenly appears...