Nanoha Takamachi, an ordinary third-grader, loves her family and friends more than anything else. One day, after having a strange dream in which a ferret gets injured, she sees the very same ferret in real life and rescues it. That ferret turns out to be Yuuno Scrya, a mage from another world who is trying to capture the 21 scattered Jewel Seeds before they cause serious damage to the universe. Yuuno is not powerful enough to capture the Jewel seeds on his own, so he grants Nanoha the intelligent device "Raising Heart" and begins training her as a mage.