Striking fear into the hearts of mortals, the Demon Lord Satan begins to conquer the land of Ente Isla with his vast demon armies. However, while embarking on this brutal quest to take over the continent, his efforts are foiled by the hero Emilia, forcing Satan to make his swift retreat through a dimensional portal only to land in the human world. Along with his loyal general Alsiel, the demon finds himself stranded in modern-day Tokyo and vows to return and complete his subjugation of Ente Isla—that is, if they can find a way back! Powerless in a world without magic, Satan assumes the guise of a human and begins working at MgRonald's to make ends meet. He soon realizes that his goal of conquering Ente Isla is just not enough as he grows determined to climb the corporate ladder and become the ruler of Earth, Whether it's part-time work or household chores this anime presents a hilarious view of the most mundane aspects of everyday life, all through the eyes of a hapless demon lord.

The once-feared Demon Lord Satan of Ente Isla, who had to flee to Earth after being defeated by the hero Emilia Justinia, is now leading a peaceful life in Tokyo under the alias Sadao Maou. Amidst a violent argument between Sadao and Emi, a dimensional portal suddenly appears, carrying a mysterious apple that harbors a toddler named Alas Ramus. To everyone's bewilderment, she imprints on Emi and Sadao, marking them as her parents! After Emi reluctantly agrees to help Sadao take care of Alas Ramus, their semblance of normal life is jeopardized by the appearance of the archangel Gabriel, who comes bearing bad news. He claims that he must take both Alas Ramus and Emi's sacred sword with him—lest the world be destroyed. Unable to accept Gabriel's grim prophecies, the hero and the former Demon Lord need to put their differences aside and unite to protect what they hold dearest.