After successfully resolving the case of the dispersed Jewel Seeds, Nanoha Takamachi resumes her regular life with added morning magic practice. She communicates with Fate Testarossa and the Arthra crew through video messages, eagerly anticipating their reunion in person. One day while studying in her room, Raising Heart alerts her of an incoming attack by a young girl named Vita who claims to be a Belka Knight. Vita, along with Shamal, Signum, and Zafila, overpowers Nanoha with a mysterious device that rapidly depletes her magic. The Space-Time Administration Bureau intervenes just in time to save Nanoha. It is revealed that the Knights are on a mission to collect magic from mages to complete the enigmatic Lost Logia- the Book of Darkness. What are Vita and her cohorts' true intentions for acquiring the Book of Darkness?