Six months following the events of the previous series, Nanoha Takamachi 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 Hayate Yagami. 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.

Magical Girl Lyrical Nanoha A's (魔法少女リリカルなのは エース, Mahō Shōjo Ririkaru Nanoha Ēsu) ("A's" is pronounced as "Ace") is a television anime series produced by Seven Arcs. It is the second anime in the Magical Girl Lyrical Nanoha franchise, following the previous series. The series aired in Japan between October 1, 2005 and December 25, 2005 and was licensed in North America by Geneon. A film adaptation, Magical Girl Lyrical Nanoha The Movie 2nd A's, was released in Japanese theaters on July 14, 2012. The series has also spawned a manga accompaniment and two video game adaptations for the PlayStation Portable. It was succeeded by Magical Girl Lyrical Nanoha StrikerS in 2007.