Two years ago, Rebecca fled her noble family in the Kingdom and became an adventurer in Yukihana, a frontier city of the Empire. For the past six months, however, her rank has stagnated and she has been unable to advance further. While investigating an abandoned church on a guild request, she meets Haru, a gentle young man who calls himself a mentor. Rumors say that every adventurer trained by the mentor has become a legend renowned across the continent. After witnessing Haru's extraordinary ability firsthand, Rebecca decides to become his disciple.

Under Haru's guidance, Rebecca begins to unlock her hidden talent for magic, shattering her own limits and preconceptions. As she grows stronger, she faces new threats: first, a never‑before‑seen aberrant monster that imperils the city, then a demon that forces her to fight alongside the deputy commander of the elite clan Covenant Cherry Blossom. Her victories earn her the title of special‑rank adventurer and the epithet Thunder Princess. Returning to Yukihana, she meets Tabasa, the granddaughter of a former disciple of Haru's, and the young noblewoman Tachiana, both of whom become new candidates for training.

The mentor’s past begins to surface. A conspiracy involving figures in black coats and the Goddess Cult threatens the continent. Haru, Rebecca, and the other disciples travel to the Labyrinth City, where Haru reunites with Hana, the master of the strongest clan Rose Garden, and trains a new generation of fighters. The scope of the conflict widens to the Imperial capital and finally to the Southern Continent. There, the true mastermind behind the cult and the black‑coated conspirators is revealed: the legendary Sword Saint, Mikazuki Tōya, a former hero who once helped save the world and later nearly destroyed it. The final battle takes place in the Kingdom of Renant during a summit of the three great powers, where Rebecca confronts her own past as well as the Sword Saint. With all of Haru’s disciples united, the mentor’s legend reaches its climax.