Due to poor finances, an epidemic, and famine, the once prosperous Tearmoon Empire ultimately falls to a revolution instigated by its citizens and supported by neighboring nations. The rebellion also brings dire consequences for Mia Luna Tearmoon, the empire's extremely spoiled princess, who endures three years in the dungeons before facing execution. By some miracle, Mia finds herself reincarnated as her 12-year-old self, retaining full memories of everything that had transpired. With the pain of the guillotine blade freshly imprinted on her neck, Mia is determined to use this second chance at life to avoid the grim fate that awaits her. She utilizes all her knowledge to improve the state of the kingdom and gather personal allies, inadvertently winning over the people's hearts and becoming beloved as a saint.

The world is divided into two kingdoms: the Kingdom of White, which floats in the heavens and is ruled by their queen Iris, and the Kingdom of Black, which stands upon desolate land below and houses the King of Darkness as its ruler. As of late, forces of evil have amassed great power, posing a threat to the entire world. Being the main representative of the Light, it is Iris' duty to maintain the balance of the world and fight off the darkness in her kingdom. Meanwhile in the Kingdom of Black, rampaging monsters annihilate a certain boy's village, leaving him the sole survivor. As he grieves in hopelessness, an armored man named Skeer notices the child and comforts him. Soon after, Skeer recognizes the boy's potential to change the kingdom's status quo and makes him his heir before passing away. The boy then vows to become the Prince of Darkness—the one who will replace the King—to bring the world back to its rightful path. As Iris and Prince of Darkness each challenge the impending doom the world faces in their own respective ways, their destinies will converge with each other, and perhaps, their bond will decide the fate of the world.

Kyou kara Maou! revolves around Yuri Shibuya, your average Japanese teenager. One day, Yuri sees a classmate being harassed by bullies. Thanks to this intervention, his friend is able to escape, but unfortunately Yuri becomes the new target of the bullies in the process and gets his head shoved into a toilet. But instead of water, the toilet contains a swirling portal that sucks him into another world, largely resembling medieval Europe. There, he is told that he will become the next Demon King due to his black hair and black eyes, traits only possessed by the demon's royal lineage. Yuri's arrival is met with some skepticism by some of the demons, who view him as unworthy to be their king. However, after Yuri wins a duel by utilizing his magical powers, the demons slowly begin to acknowledge him as their monarch. Yuri must now learn what it takes be a true Demon King, as he tries to keep the peace between demons and humans in this strange new realm.

Prince Lloyd de Saloum—once a commoner who could never grasp magic—is now blessed with inexhaustible mana and has all the time in the world to master it, as he has no claim to the throne. After mastering demonic magic, he sets his sights on the Church to learn the secrets of divine magic that will bring him one step closer to completing his arsenal. However, due to his fondness for unholy magic, he draws the scrutiny of divine beings who see him as little more than a demonic spawn.