In another dimension where magic exists, the Dark Lord Satan wages a campaign with his four generals, Adramelech, Lucifer, Alciel and Malacoda to conquer the multiple-island continent of Ente Isla and annex the human population under his realm of darkness. However, a brave Hero arose and lead the human armies against the Dark Forces, pushing them back to their stronghold. Here, Satan faces off against the Hero but eventually admits defeat, and instead, along with Alciel, opens a dimensional portal and leaves their world with a vow to return. The portal exits in modern-day Sasazuka, Japan, and in addition to being shocked by their new human forms, Satan and Alciel are taken to the nearest police station, where Satan learns a bit about this new world, which lacks magic. As events progress, Satan and Alciel, get new identities: Sadao Maō and Shirō Ashiya respectively, secure a residence, open a bank account and go job hunting in order to survive, while agreeing that Sadao would work while Shirō takes care of their domestic needs. Some time later, on his way to his part-time job at a MgRonald franchise, Sadao encounters a beautiful scarlet-haired girl at a crosswalk and offers her his umbrella to shield from the rain. While at MgRonald, Sadao not only experiences a seemingly random mild earthquake, but faces a dilemma on whether to use his remaining magic to repair the broken fryers, eventually not getting the chance to. Nonetheless, while impressing his fellow co-worker Chiho Sasaki with his people skills as well as the higher-ups through his dedication, Sadao is able to secure a permanent position. As he excitedly pedals his bicycle home to tell Shirō, he is once again confronted by the scarlet-haired girl who is revealed to be the Hero from their dimension, Emilia Justina.