Magical Girl Lyrical Nanoha StrikerS' story happens four years past the JS incident, and its protagonist is Vivio Takamichi, a fourth-grade pupil at St. Hilde Academy of Magic. Vivio has honed her magical abilities to transform into an adult form and has been granted her very own Device, Sacred Heart, by Nanoha. Moreover, she has pledged not to utilize them for any mischievous deeds. However, one day, a strange fighter with magical abilities sets its sights on Vivio.