Following the resolution of the Jewel Seeds incident, Nanoha Takamachi resumes her daily routine intertwined with magical training. Communicating with Fate Testarossa and the Arthra crew via video messages, Nanoha yearns for their next face-to-face meeting. Unexpectedly, during a study session in her room, Nanoha is alerted by Raising Heart to an imminent attack. The assailant, Vita, introduces herself as a Belka Knight and showcases her prowess with a mysterious cartridge-based device, swiftly overpowering Nanoha. Fortunately, intervention from the Space-Time Administration Bureau prevents Nanoha's defeat. Vita, along with her comrades Shamal, Signum, and Zafila, are on a quest to drain magical energy from mages to fulfill the Book of Darkness, a coveted Lost Logia. What dark intentions drive the knights to pursue this artifact?