The manga takes place four years after the JS incident in Magical Girl Lyrical Nanoha StrikerS, and stars Vivio Takamichi, a fourth-year elementary student at St. Hilde Academy of Magic. Vivio has learned to conjure her adult form and has received her own Device — named Sacred Heart — from Nanoha, and has promised Nanoha to never use either for mischief. Then a mysterious magical fighter appears and targets Vivio.

(Source: Anime News Network)