The story centers around Nanoha Takamachi, played by Yukari Tamura, a young nine-year-old Japanese girl who leads a quiet, ordinary life attending elementary school alongside her parents and older siblings. However, her mundane routine is completely transformed when she discovers and rescues an injured ferret. To her surprise, the animal reveals itself to be a young shapeshifting mage named Yūno Scrya, voiced by Kaori Mizuhashi.
Yūno is an archaeologist hailing from a parallel dimension known as Midchilda. He has traveled to Earth on a critical mission: to recover a collection of 21 ancient and perilous artifacts known as the "Jewel Seeds." These artifacts possess the dangerous ability to grant unnatural powers to any living being that comes into contact with them, though this power often results in the creature mutating into a mindless monster. Because Yūno was injured during his attempts to secure these seeds, he is forced to remain in his ferret form to recover, relying heavily on the kindness of Nanoha. During this time, he provides her with an intelligent device—a magical wand named Raising Heart. To Yūno's amazement, Nanoha displays an incredible and unexpected aptitude for the arcane arts. While maintaining her facade of a normal schoolgirl, she begins training in magic under Yūno's guidance as they work together to track down the scattered seeds.
The plot intensifies when Nanoha attempts to retrieve her sixth Jewel Seed and crosses paths with another magical girl, Fate Testarossa, voiced by Nana Mizuki, and her loyal familiar, Arf, voiced by Natsuko Kuwatani. It is revealed that Fate's life is defined by tragedy; over a year prior, her mother, Precia Testarossa, suffered a mental breakdown following the death of her daughter, Alicia. This led to the creation of "Project Fate," an illegal and unethical research operation focused on cloning and resurrection. As a result, Precia became a fugitive, hunted by the Time-Space Administration Bureau (TSAB), the inter-dimensional police force.
Fate is a clone of Alicia, implanted with the original girl's memories. Despite being burdened with these happy childhood memories, Fate suffers constant abuse from Precia, who is unable to love the clone as she loved the original daughter. Driven by a desperate loyalty to her mother, Fate is coerced into collecting the Jewel Seeds to power a gateway to Al Hazard, a mythical realm where Precia believes Alicia can be truly resurrected.
As the story progresses, Nanoha and Fate engage in repeated, high-stakes magical confrontations over the discovery of each new Jewel Seed. The scale of their battles eventually draws the attention of the TSAB, who intervene to prevent further collateral damage to the environment. Eventually, Nanoha manages to defeat Fate in combat and delivers her into the custody of the TSAB. Upon realizing that Fate has been captured, Precia heartlessly abandons her daughter, attempting to use the seeds already in her possession to perform a dimensional jump to Al Hazard.
Finding herself betrayed and alone, Fate finds the strength to resolve her inner conflict and decides to ally herself with the TSAB and Nanoha to stop Precia's madness. Although they work tirelessly to minimize the destructive side-effects of the magic being used, they are unable to stop Precia from completing her spell. As the climax concludes, Precia's final location remains a mystery. In the aftermath of the conflict, Nanoha and Fate forge a deep and lasting friendship, though Fate must first travel to Midchilda to face the TSAB and prove that she was merely an unwilling pawn in her mother's criminal schemes.
