The story line follows Nanoha Takamachi, an everyday nine-year-old Japanese girl. Nanoha's regular everyday 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, Yūno came to Earth to collect a set of twenty-one dangerous ancient artifacts named the "Jewel Seeds" 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, and Yūno, injured while trying to collect them, must now rely on Nanoha while he convalesces in ferret form.

(Source: Wikipedia)