Born with a deadly curse sealed in his left chest, Prince Callus Leditzweissen of the royal family is a Taboo Being whose body is wracked with constant, agonizing pain. By the age of ten, doctors and mages have exhausted all remedies, and he is given only six months to live. Bedridden and resigned to a short, suffering existence, his fate shifts when the sage Gourley arrives at the palace. Gourley demonstrates that light magic, the only power capable of purifying darkness, can temporarily suppress the curse's torment. Seizing this glimmer of hope, Callus begs to become Gourley's apprentice, determined to master the legendary art himself within the scant time he has left. Despite the near-impossible odds, Gourley agrees, and Callus begins an intense crash course in light magic, discovering that his cursed body paradoxically holds a vast wellspring of blessed magical energy.

As Callus trains alongside his loyal maid Shizuku and under the watchful eye of his supportive family, he slowly regains the strength to move beyond his sickbed. His brothers, the sword master Sieg Lamiared, and Sieg's spirited daughter Crys become part of his expanding world. He befriends the light spirit princess Serena, who aids his studies, and later, when his progress stalls, Gourley calls upon his shy but prodigious disciple Sissy. Sissy brings a rare and ancient tome that contains crucial secrets about the nature of Taboo Beings and their curses. Through grueling practice, dangerous encounters with monsters like wyverns, and a growing circle of allies, Callus pushes his body and magic to their limits. He eventually earns a place at the Academy of Magic, only to find new political intrigues and a mysterious moon magician who seems to know more about his condition than she reveals. The deeper Callus digs into the curse, the more he uncovers a web of ancient grudges and divine meddling, forcing him to fight not only for his own survival but to protect the people who have given him a reason to live.