The storyline follows Nanoha Takamachi, a nine-year-old Japanese girl attending elementary school, who lives with her parents and her older siblings. Nanoha's regular daily life ends when she rescues an injured ferret who reveals himself as a young shapeshifting mage named Yūno Scrya. An archaeologist from a parallel universe called Midchilda, Yūno came to Earth to collect a set of 21 dangerous ancient artifacts named the that he first discovered in his own world. Jewel Seeds give living beings who come into contact with them unnatural powers, often turning them into monsters. Yūno, injured while trying to collect them, must now rely on Nanoha while he convalesces in ferret form. He gives Nanoha an "intelligent device" (magical wand) called , and she unexpectedly shows strong aptitude for magic. As the two gather the Jewel Seeds, Nanoha learns magic from Yūno while continuing with her ordinary everyday life.