Months after the Jewel Seed incident, Nanoha Takamachi prepares to welcome Fate Testarossa back from her trial with the Administrative Bureau. Fate received conditional clemency for her role in stopping the crisis and has been permitted to attend school alongside Nanoha. But before the two girls can settle into their routine together, a band of dark mages suddenly attacks. Overwhelmed by the unknown assailants, Nanoha and Fate have their mana drained into a sinister grimoire, and both Raising Heart and Bardiche are damaged.
The attackers are the Wolkenritter, a group of four—Vita, Signum, Shamal, and the familiar Zafila—summoned by the Book of Darkness to carry out its evil agenda. The book's current owner is a young girl named Hayate Yagami, who remains unaware of its true nature while the Wolkenritter, compelled to charge its power with forcibly stolen mana, are bound to complete the tome.
Left exhausted and without their Devices, Nanoha and Fate must quickly regain their strength and face the Wolkenritter, hoping to stop them from finishing the Book of Darkness and unleashing its devastating magic upon the world.