Humanity has successfully colonized all the planets in the Solar System, with intentions to continue into new frontiers. Individuals live stable lives in these colonies, including young high school students living in 4th Tokyo on Mars. Two of these students are Iris Shirazaki and Mizuki Sera. Iris is finishing her last year of high school while studying to be a test pilot while her friend Mizuki also works as a mechanic on the aircraft. Her own brother, Kaito, oversees Mizuki and Iris as their homeroom teacher and program director.
Dreaming of becoming a hero and vanquishing the Demon King, Raul Chaser enters the Hero Training Program in pursuit of his ambition. However, when the Demon King is defeated and peace returns to the world, the Hero Training Program is suspended indefinitely, making it impossible for anyone to become a hero.
Two years later, Raul reluctantly works at a small electronics store called Magic Shop Leon. Though the former hero-in-training is plagued by the mundanity of working in retail, everything changes with the arrival of a new hire. Appearing at first to be just a boy with good looks, "he" turns out to be a female demon by the name of Fino Bloodstone. She is not just any old demon either—Raul's new coworker is in fact the daughter of the late Demon King! Handed the responsibility of training this eccentric new employee, Raul soon finds his life becoming livelier than it ever was before.