The story begins with Tanaka Shogo, a salaryman, experiencing a sudden dizzy spell after leaving Kanda Station. He finds himself in a dark, stone-walled room, reminiscent of a horror movie basement. A cloaked man appears, stating he has finally found Shogo and intends to swap places with him, bestowing some knowledge before vanishing into particles that absorb into Shogo. Disoriented, Shogo discovers another room filled with antique-like items and a skeleton in metal armor. To his shock, the skeleton, named Aperanza, speaks, revealing Shogo is its new master and that he has been sealed. Shogo's mind is flooded with memories of his past life as a demon, sealed away by humans.
Upon waking, Shogo confirms his demonic past and the circumstances of his sealing: his home was attacked by humans, and he fled into a basement that was sealed along with him. He recalls a long, arduous process of finding a way to escape by summoning a being with a similar magical wavelength from another dimension to swap places. Aperanza confirms Shogo's knowledge, explaining that his former master, a demon, had lived peacefully, forming a village, until humans attacked. Shogo questions the common perception of demons as overwhelmingly powerful, and Aperanza clarifies that demons primarily possess more magic and longer lifespans than humans, though they are otherwise similar in appearance and vulnerability.
Shogo learns that escaping the sealed basement requires destroying a key, a task his predecessor couldn't accomplish. Days turn into years, marked by the slow descent of sand in a three-year hourglass, indicating over a century has passed. Shogo, bored and facing dwindling magical power, practices by throwing magic bullets at a wall. He eventually discovers a way to tame moles using magic, creating pets he names Mogutan. He commands the moles to search for the seal's key. After a considerable time, the moles bring a broken stone tablet, inscribed with sealing runes. As Shogo touches it, the seal breaks, and his magic rapidly replenishes. Aperanza, his loyal servant, disintegrates into dust as the seal's magic dissipates.
Shogo emerges into a desolate wasteland, a stark contrast to his memories of a lush mountain. He bids farewell to the moles and sets out, encountering a group of merchants attacked by bandits. He intervenes, effortlessly dispatching the bandits with his magic. The merchants, led by Runos, are grateful and mistake him for a powerful mage. They offer him passage to the city of Bosani. During their journey, Shogo overhears the guards discussing him, speculating he is a mage fleeing the war-torn Benda Empire and possibly a "demon."
Upon arriving at Bosani, a heavily fortified city, Shogo obtains a temporary entry permit. He secures lodging and, while eating, Runos and his guards propose he accompany them on a dangerous supply run to a northern fort near the Benda border. Shogo, suspecting they might be using him as a pawn due to his perceived connection to Benda, agrees to decide the next day. However, Benda Empire soldiers suddenly storm the inn, arresting Shogo, who they believe is a Benda mage. He is taken to a dark, foul-smelling prison cell.
Using his magic, Shogo frees himself and a fellow prisoner, a muscular man named Est, a lieutenant from the Benda Empire's 18th Division. They escape through the sewers and, after a brief encounter with pursuers, find themselves by a river. Est reveals his unit was left behind during a failed invasion and that he wishes to return to the Benda Empire. Shogo, still harboring the knowledge of his past life as a demon and the existence of a teleportation magic, proposes a risky plan to use a specific location to return to his original world. They head towards the western forests of Bosani, hoping to evade pursuit. Shogo then uses his knowledge of teleportation magic, intending to return to the familiar underground chamber where he was sealed.