Sadao Maou, the former Demon Lord Satan of Ente Isla who fled to Earth after being defeated by the hero Emilia Justinia, now lives peacefully in Tokyo under that alias. As a model employee at a local fast-food restaurant, Sadao supports his former generals, Alciel and Lucifer, who have joined him in Japan, while avoiding confrontations with Emi Yusa—Emilia's assumed name—and the angels monitoring his actions. During a heated argument between Sadao and Emi, a dimensional portal suddenly opens, delivering a mysterious apple containing a toddler named Alas Ramus. To everyone's surprise, she imprints on Emi and Sadao, identifying them as her parents! After Emi reluctantly agrees to help Sadao care for Alas Ramus, their semblance of normal life is threatened by the arrival of the archangel Gabriel, who brings bad news. He claims he must take both Alas Ramus and Emi's sacred sword—lest the world be destroyed. Unable to accept Gabriel's grim prophecies, the hero and the former Demon Lord must set aside their differences and unite to protect what they hold dearest.