On the Holy Cross Continent in a parallel space, the evil demon king Satan (voiced by Ryota Oosaka) sent out demon troops to invade the surrounding territories. Humans, led by the warrior Emilia (voiced by Yoko Hikasa), rose up in resistance and eventually broke into the Demon Palace, forcing Satan to flee. When the demon king regained consciousness, he found himself in a completely unfamiliar Earth, Tokyo, Japan, with his loyal follower Arsenal (voiced by Tomoki Ono). The two who had lost all their magic power had to collect social security cards, bank cards and other necessary items for their livelihoods, and had to go out to work every day to maintain their food, clothing and daily necessities. The demon king, who was always eager to return to his homeland, accidentally discovered that Emilia had also tracked him to Earth one day! Although food, clothing, housing and transportation were annoying, these three young people who seemed to be plagued by the second disease gradually got used to life on Earth. At the same time, it seemed that there was another force lurking around them, ready to erase the existence of the demon king and the brave at any time... This film is adapted from the light novel "The Demon King's Castle is Six Mats and One Room!" by Satoshi Wagahara.