Jack, a middle-aged deliveryman living an ordinary and uneventful life, is suddenly summoned to the royal capital without explanation. There, he is met by Riesellet, a young woman he once transported as a package years ago who has since grown into the captain of the knight order. She informs Jack that he has been nominated for the title of One Sword, a legendary rank reserved for the truly strongest swordsman in the kingdom. Unaware of the immense power hidden within himself, Jack reluctantly accepts the role and soon discovers that his new position is inextricably tied to the looming threat of the Demon King. As he begins to train and face increasingly dangerous adversaries, Jack realizes his mundane delivery job has somehow become the cover for a side career as an invincible warrior. The story follows his journey from a lackluster freight employee to humanity's unexpected trump card against the demon army, confronting powerful enemies such as the Seven Succession Calamity Stars, including the formidable Bougain and Victor. Along the way, Jack takes on disciples and fights to protect villages under demon attack, all while grappling with his own unassuming nature and the weight of his newly awakened abilities.