A young woman named Spica Virgo dreams of becoming a great sorceress and enrolling in the prestigious Royal Diana Academy, but she cannot use magic at all and lacks the money or connections to find a mentor. Her fortunes change when she encounters a mysterious black cat who can speak and wield powerful spells. The cat, who reveals himself to be the legendary but disgraced sorcerer Claude Sirius, was cursed into feline form two years earlier during a terrorist attack by a heretical organization known as the Black Serpents. Claude had become the academy's youngest instructor at seventeen but vanished after the incident, leaving his reputation in ruins. Spica convinces Claude to take her on as his secret apprentice, agreeing to his one condition: she must grow strong enough to break his curse and restore his human body. The key to temporarily lifting the curse proves to be a kiss on an embarrassing location, leading to frequent awkward moments as Spica works to improve her abilities while helping Claude regain his true form.

The story is set primarily at the Royal Diana Academy, a prestigious boarding school in the kingdom of Aladia that accepts only the most gifted magic users. Students who graduate from the academy go on to become elite sorcerers serving the royal family or working at the highest levels of their fields. Upon enrolling, Spica is placed in a special class known derisively among the faculty as Pandoras Box, a group of problematic but talented students placed under Claudes supervision after he returns to teaching in his human form. This class, officially called the Claude Coven, brings together a colorful cast of characters. Among them are Aria Aquarius, Spicas childhood friend and rival who excels at water-based magic; Io Taurus, a gentle giantess with immense physical strength; Astraea Libra, the flirtatious son of the vice-headmaster; Castor and Pollux Gemini, twins who share a single body and switch based on who touches them; Leo Regulus, a muscular animal transformation specialist; Capella Capricorn, who wields anti-magic abilities; and Ewe Aries, a sleepy fire magic user.

The narrative unfolds across multiple arcs that follow Spicas growth from a magicless apprentice to a capable sorceress. After passing the entrance examination with Arias help, Spica must navigate the challenges of academy life while keeping her secret pact with Claude hidden. The class participates in various trials, including a midterm examination that involves navigating a griffins nest and a winter training camp on Snake Turtle Island. Throughout these events, Spica gradually unlocks her unique magical talent: a rare and powerful form of magic called Rebirth magic, which belongs to the Ophiuchus zodiac and grants her the ability to heal injuries and break curses. This power makes her a target for the Black Serpents, the same organization that cursed Claude, who seek to obtain her abilities for their own purposes. The central conflict escalates as Claude works to protect his students while seeking revenge against those who destroyed his reputation, culminating in a confrontation with the Black Serpents at the royal palace where Spica fully awakens her powers.