Mahou Shoujo Lyrical Nanoha A's

After resolving the incident involving the scattered Jewel Seeds, Nanoha Takamachi happily returns to her everyday life, now adding morning magic practice to her routine. She exchanges video messages with Fate Testarossa and the crew of the Arthra, eagerly awaiting the chance to meet them in person again. One day while studying in her room, Raising Heart calls out to Nanoha and warns her of an incoming attack. The attacker is a young girl named Vita, who calls herself a Belka Knight. She proves her strength by using an intelligent device equipped with a mysterious cartridge system to quickly overwhelm Nanoha. Fortunately, the Space-Time Administration Bureau intervenes before she is completely crushed. Vita, along with her fellow knights Shamal, Signum, and Zafila, is on a mission to steal magical power from mages in order to complete the Book of Darkness, one of the Lost Logia. For what sinister purpose are the knights pursuing this Book of Darkness?