In a continent that lies in the ocean of Ignora is Ente Isla. Maou Satan, or Dark Lord Satan, wants to conquer it.
To do it, he declared war against the human world to annex its population under his realm of darkness. He is helped by his four generals, Adramelech, Lucifer, Alsiel and Malacoda.
Unfortunately for them, a Hero, named Emi Yusa, arose from within the humans to lead their army against the Dark Lord Satan. First, they took Lucifer in the west, followed by Adramelech in the north and Malacoda in the south. In the end, they were pushed back to their stronghold and put under siege.
Satan faces off Emi, but eventually admits defeat and grants the humans their victory. Satan opens a dimensional portal and leaves alongside Alsiel to another time and space.
The portal exits in a modern Japan, where Satan and Alsiel will have to find a safe place to recover. But first, they have to face the fact that they have changed into humans. Now, they will have to face police and the whole idea that this world is different.
So, Satan and Alsiel are taken to the nearest police station. Here, Satan begins to learn about this new world, where magic, demons and Dark Lords are just a myth.
Now, the Dark Lord Satan faces a world without magic, and most of all, they have to understand that magic will be difficult to replenish, but still, they believe there is a chance to do it. Before that, they will have to live under the human world rules and do what is needed to return to their world.
With a little magic, Satan and Alsiel get new identities and name themselves as Sadao Maou and Shirou Ashiya respectively, then, they are able to secure a residence, open a bank account and go job hunting in order to survive.
Sometime later, on his way to his part-time job at a MgRonald franchise, Sadao encounters a girl at a crosswalk and offers her his umbrella to shield from the rain, he has no idea, but she is an old acquaintance.
While Sadao works at MgRonald, Ashiya takes care of their domestic needs as well as he researches means to replenish their magic powers and return to Ente Isla.