On the magical world of Midchilda, Vivio Takamachi is a 10-year-old girl who lives with her adoptive parents, Nanoha Takamachi and Fate T. Harlown, and goes to school with her friends Corona Timil and Rio Wesley. To celebrate Vivio entering her fourth year of elementary school, Nanoha decides to give her her own rabbit-shaped Intelligent Device, which she decides to name "Sacred Heart" or "Chris" for short. Using her new device, Vivio transforms into her mage form, taking the adult Sankt Kaiser form she previously used during an incident several years ago, which comes as a surprise to Fate. As Vivio does some late night training with Nanoha, her Strike Arts instructor, Nove Nakajima, hears about a girl going under the name of the Sankt Kaiser Heidi E. S. Ingvalt who has been attacking various people. The next day, after Nove accompanies Vivio to visit her comatose friend, Ixpellia, she is challenged by the self-proclaimed Ingvalt.
高町ヴィヴィオは10歳。ミッドチルダ在住のSt,ヒルデ魔法学院４年生。大親友のコロナ・リオと一緒に、勉強やスポーツ、色々な事に挑戦中。特に今は、師匠のノーヴェに教わりながらストライクアーツという格闘競技のトレーニングに汗を流している。しかし、平和な街に、格闘系の実力者を狙った連続傷害事件が頻発しているとの知らせが、ナカジマ家に届く。そんなある日、ノーヴェの目の前に、『覇王』を名乗る連続傷害犯が現れた。
