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. Having become a model employee of a local fast-food restaurant, Sadao has to provide for his former generals Alciel and Lucifer who joined him in Japan, as well as avoid confrontations with Emi Yusa—the assumed name of Emilia—and the angels who monitor his actions.

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.