The narrative begins with Usato Ken, a seemingly detached observer in a chaotic medical tent filled with the sounds of pain and rough laughter. Usato, also known as Usato, is the vice-commander of the Rescue Corps, possessing powerful healing magic. Despite the grim surroundings, he finds a strange sense of contentment in his work, though his subordinates' boisterous methods often require his stern correction. Usato reveals he was summoned to this world as an "extra" to a hero, a consequence of a series of unfortunate events.
The story flashes back to a rainy day in Usato's original world. While waiting for the rain to stop, he encounters Kazuki Ryusen, a popular and perfect student council member, and Suzune Inugami, the elegant and capable student council president. A chance encounter leads to Usato becoming friends with them, a rare and exciting event for him. As they walk home, they discuss their futures, with Suzune expressing a lack of direction despite her talents. Their conversation is interrupted by a strange bell sound, and a magic circle appears beneath them, transporting them to another world.
They awaken in a grand hall before King Lloyd of the Ringl Kingdom. He explains they have been summoned as heroes to combat a resurgent Demon King. Kazuki and Suzune are identified as having the necessary "aptitude" based on the bell sound, but Usato, who couldn't hear it, is revealed to be an unintended casualty of the summoning. Despite his accidental arrival, King Lloyd cannot send him back. Kazuki, fiercely loyal to his new friends, demands answers and a way home. The King, humbled, pleads for their help.
Usato, Kazuki, and Suzune are then subjected to aptitude tests using a crystal. Kazuki displays aptitude for light magic, and Suzune for lightning magic, both with considerable magical power. Usato, however, is revealed to have healing magic, a rare and often undervalued attribute. The King and his advisors react with a mixture of confusion and concern, suggesting Usato be sent to a town doctor for training, a decision that seems to hide something.
Usato is then taken by Rose, the formidable and intimidating leader of the Rescue Corps, to a secluded medical facility. There, he is introduced to Rose's subordinates: a group of rough-looking men who greet him with unsettling enthusiasm. Rose informs Usato that he will be staying and training there, and that his training will be under her direct supervision.
Usato's training begins, described as hellish. He endures grueling physical conditioning, including extreme running and push-ups, often with weights, while simultaneously practicing his healing magic to recover from his injuries. He learns about the world through books, discovering the existence of monsters and various races. His companions, Kazuki and Suzune, are receiving more conventional hero training at the castle.
After a month of intense training, Usato's physical abilities have dramatically improved. Rose takes him to a dangerous forest known as "Ringl's Darkness" and tasks him with hunting a Grand Grizzly, a powerful monster. He is thrown into the forest with a massive backpack filled with survival gear. He encounters two Blue Grizzlies and a Grand Grizzly, but before he can engage them, they are brutally killed by an unknown, powerful entity. Usato is enraged by the destruction and the fact that he cannot prove he completed his task.
He then encounters a young Blue Grizzly, seemingly orphaned by the attack. Despite his initial frustration, Usato heals the cub and, after it leads him to the lair of the remaining monsters, he resolves to defeat the creature responsible for the carnage, vowing to avenge the fallen and confront the powerful entity, implied to be a giant snake-like monster called a Gigano-snake.
Meanwhile, Kazuki and Suzune have become significantly stronger. Suzune, dissatisfied with the "lack of romance" in her magic, seeks more creative applications. They learn of rumors about Usato's extreme training and decide to visit him. They find him performing an impossibly difficult set of push-ups with Rose sitting on a boulder on his back. Sirgulus, a knight commander, intervenes, angered by Rose's harsh methods, but Rose asserts her training is necessary to make Usato her "right arm."
Usato, having survived his ordeal in the forest, has become more resilient and even somewhat natural in his harsh environment. He has developed a close bond with the black rabbit that guided him, and has begun to understand the true nature of his healing magic, realizing its necessity for survival. He is determined to defeat the monster that killed the Grand Grizzly and prove himself, not just to Rose, but to himself.
