Several months have passed since the Jewel Seed incident, and Nanoha Takamachi is preparing to welcome Fate Testarossa back from her trial with the Administrative Bureau. She has received conditional clemency for her role in preventing the crisis, allowing her to attend school with Nanoha. However, before they can settle into their daily lives together, a group of dark mages suddenly attack them. Overpowered by these unknown assailants, Nanoha and Fate have their mana absorbed by an ominous grimoire, and both Raising Heart and Bardiche are damaged in the process.
The perpetrators behind this ambush are known as the Wolkenritter, consisting of four members: Vita, Signum, Shamal, and Zafila. The current owner of the Book of Darkness is Hayate Yagami, who remains oblivious to its true nature while the Wolkenritter charge their power with the forcibly stolen mana.
Left drained and weaponless, Nanoha and Fate must race against time to recover their strength and confront the Wolkenritter in order to prevent them from completing the Book of Darkness and unleashing its devastating magic upon the world.