Shaped by experiences that left him weary of human relationships, Rei Hitoma openly dislikes people. Seeking a quiet, uneventful life, he accepts a teaching position at a secluded school in the mountains, only to discover that his classroom is anything but ordinary. The students under his care are demi-humans who have gathered there with a singular goal: to learn what it means to become human. Despite his disillusionment, Rei finds himself responsible for explaining human behavior and social norms to mermaids, bunnies, werewolves, and the like. As his students earnestly struggle to understand humanity, their questions and aspirations gradually expose the contradictions in Rei's own beliefs.

En route to conquer a Great Labyrinth with his allies and former classmates, Hajime Nagumo is sidetracked by a tragedy that befalls his party member Shea Haulia. After encountering a group of rabbitmen caught in an ambush, Nagumo learns that members of Shea's Haulia Tribe have been abducted by soldiers of the Hoelscher Empire. To alleviate the anxiety of Shea, whose father may be in mortal danger, Nagumo decides to head toward the empire and rescue the missing Haulias. After arriving at his destination, Hajime is caught up in political turmoil between the empire and the Heiligh Kingdom. As princess Liliana S. B. Heiligh tries to formalize an alliance with the empire to protect her weakened kingdom against demon attacks, the Haulia Tribe plans to assassinate Emperor Gahard D. Hoelscher to free all rabbitmen from the empire's threat. Determined to protect his allies at any cost, Hajime will need to ignore all political considerations and unleash his unstoppable power on anyone who may stand in his way.

After rescuing his former classmates from the perils of a labyrinth—a mythical place that grants a divine magic to whosoever triumphs over its trials—Nagumo Hajime emerges victorious, while the new heroes of the fantasy world Tortus must face their first major failure. Together with his traveling companions and the new team member Kaori Shirasaki, Nagumo resumes his journey to conquer all labyrinths and eventually return to his original world. Meanwhile, the heroes from Earth try learning from their mistakes and prepare for the demon army's invasion. As the final confrontation looms closer, new and old enemies start setting their evil plans in motion. Once more, Nagumo must prove that he can prevail against the most formidable opponents, all while protecting the ones he holds closest to his heart.

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.