From Marvelous Entertainment (Mushi-shi, Air Gear) and Artland (Mobile Suit Gundam 00, Legend of the Galactic Heroes) comes the story of young Akuto Sai, a new student at Constan Magical Academy. On the day of admittance, his aptitude test has predicted his future occupation as... Demon King. And so begins the trials and tribulations of school life as he is hounded by do-gooders, self appointed protectors, and overzealous followers. In this school, there's no rest for the Devil King.
Akuto Sai transfers to the Constan Magical Academy with high hopes of becoming the Grand Priest. Like all new students, he must undergo an evaluation, which predicts that his future occupation will be as the Demon King. It’s hard enough to be the new kid, but having the entire school either run in terror or try to destroy you can really make high school life the pits!
Akuto meets Fujiko Eto, the female dorm leader and the only person who seems to understand him at school. He wants to meet with her in private, but Korone, the emotion-analyzing government-issue android, and the gossip-minded student body keep him under constant watch. Akuto must travel to a dangerous rendezvous point and find a way to ditch Korone on the way.
Fujiko has set up a meeting underground between Akuto and Junko, so he can try to repair the damage to their relationship. Once again, he has to get past the annoying Korone – but this time, Kena Soga shows up, wanting Akuto to make good on his promise to be friends. When he finally gets away, he encounters a nasty surprise waiting for him in the underground.
After Akuto demolishes the practice yard during what should have been an easy magic lesson, Mitsuko suggests he spends time in the mental discipline chamber in order to learn to control mana. Although he is meant to go alone, Korone and Kena accompany him, as Fujko appears outside with a scheme to gain his loyalty. Things don’t go as planned, but they come away with a mysterious map that Kena believes will show the way to pirate treasure.
Akuto, Eiko, Hiroshi and Korone enter the underground graveyard beneath the charnel house, where they find evidence of past battles. Junko, Kena and Fujiko are hot on their trail, trying to reach Akuto before he discovers the true nature of the map’s treasure. But Fujiko isn’t there for Akuto’s sake – she is following the same trail her brother took when he died, hoping to discover what caused him to flee in cowardice and destroy their family name.
The students head to the beach on a school trip. Korone’s teasing of Akuto has intensified lately, resulting in publicly exposing Junko. Akuto chastises the android, leaving Korone strangely affected by his scolding. Hiroshi is also acting strangely – his family lives on the island, which harbors a legend about a hero arising from the island’s populace who could defeat the Demon King and the monster that appears in the island’s lake when the Demon King is reincarnated.
It turns out that Akuto isn’t the only one who received a huge surprise for their future job – Hiroshi was predicted to be the island’s legendary hero. But, just like Akuto, he rejected the idea, although his parents and younger sister were thrilled. Yukiko wants confirmation from Akuto that is the Demon King, and when he denies this, she runs away. While searching for her, Kena and Hiroshi run into a strange man at the lake who is far worse than the pervert he first appears to be.
Demon beasts have been unsealed around the country, but fortunately Brave, the legendary hero, has appeared to defeat them. The beasts underground Constant Academy are also awake, and Fujiko takes an egg hoping to tame the beast inside. Meanwhile, Akuto tries to get Kena to attend classes, but she refuses. Korone explains that other students look down on her, as she cannot use magic – a sign that she is ill-favored by the mana-controlling computer, God. Akuto is angered by this revelation.
The school session has ended for a long break, and Junko asks Akuto to come with her to her family’s house. Akuto misunderstands why he is at the home of the Iga ninja clan and winds up unknowingly accepting the marriage – to Junko’s younger sister’s displeasure. Meanwhile, it seems there is a scheme working against Kena, who was left alone at the Academy.
Fujiko’s beasts tear up the Academy as they run from the fight with Yamato, hoping to get Akuto to his weapon, Peterhausen. The Koga and Iga ninja clans unite to fight the Demon King under the newly-appointed Teruya leader, Eiko. Yuko and Junko are sent alone to fight the beasts. Akuto comes to understand that, in order to protect Kena, he must make her truly free – and that means eliminating the mana-generating and mana-controlling computer, God.
Akuto takes to the air, atop Peterhausen, with one goal in mind: the only way to protect Kena is to destroy God! The enemy battleship "Black Crane" makes a surprising entrance and halts Akuto's plan of Deicide. Meanwhile, Eiko and her army storm the school. Bouichiro makes his presence known once again, executing his own plans to the tee.
Bouichiro's plan is coming to fruition: he has successfully made it to the Suhara Shrine, Kena in hand, and plans to make the contract with the Law of Identity to finish off his masterpiece. Akuto appears, fully healed, and the final battle ensues! Akuto learns the ultimate truth of his existence and must make a life altering decision.