Once a feared Demon Lord, Satan finds himself living a peaceful life in Tokyo as Sadao Maou after escaping defeat by the hero Emilia Justinia of Ente Isla. Balancing his job at a fast-food joint, Sadao must support his former generals Alciel and Lucifer in this new world while navigating encounters with Emi Yusa (Emilia). A sudden portal disrupts their routine, delivering an apple containing a toddler named Alas Ramus, who unexpectedly binds herself to Emi and Sadao. Despite initial resistance, Emi joins forces with Sadao to care for Alas Ramus, but their tranquility is short-lived as the archangel Gabriel arrives with dire warnings. Threatening to take Alas Ramus and Emi's sacred sword, Gabriel forces the unlikely allies to protect their newfound family and world from impending doom, setting aside their past animosity to confront a common enemy.