The 13-episode Magical Girl Lyrical Nanoha A's is the sequel to the anime series Magical Girl Lyrical Nanoha. This time round we have the main antagonist in the original series, Fate joining Nanoha in her fight against the mysterious knights of The Book of Darkness. Similar to the first story, we soon learn that the knights aren't evil. They're simply misguided and only wish to save their mistress who will die if they don't steal the magic required by the Book.
Nanoha's story isn't much to shout about, either the first or this second season. The characters are too cliched. Nanoha is a sweet and kind girl who wants to help those around her while Fate wants to show the Knights and especially Signum the error of their ways, having regretted her past actions. It's hard to dislike any of the characters but I don't feel any particular affection for them either. And throughout the series I couldn't help comparing Nanoha to Sakura of Card Captor Sakura. I might have liked Nanoha better if I'd never seen Sakura. As it is, Sakura wins simply she was there first.
The only interesting part of the story is how magic is combined with science fiction in the story. The mage's staffs in the series are almost like AI which operate both on voice command and upon the unspoken wishes of their masters. They even sometimes make decisions on their own and care about their masters well-being.
It's worth at least a watch, but doesn't bear repeating unless you've already forgotten the story.