Six months later, Takamachi Nanoha and Fate Testarossa have been exchanging video mails to tell each other of their situations on Earth and in the Time-Space Administration Bureau respectively. However, on the night Fate returns, Nanoha and her friends come into conflict with the Belkan Knights, Signum, Vita, Zafira, and Shamal, who are tasked with filling the pages of the Book of Darkness to protect their master, a wheelchair-bound girl named Yagami Hayate.

It is up to Nanoha, Fate and the Time-Space Administration Bureau to solve the mystery of the Book of Darkness, the Belkan Knights and their master.

Source: Wikipedia

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Prequel: Mahou Shoujo Lyrical Nanoha
Sequel: Mahou Shoujo Lyrical Nanoha StrikerS
Alternative version: Magical Girl Lyrical Nanoha The Movie 2nd A's