Shortly after the end of season one, before Yuuno leaves, Nanoha decides that she wants to go visit Fate. After whining, bribing, and outright threatening Yuuno he finally caves and tells her how to set up a dimensional jump.
Sadly even a mage of Nanoha's raw power is still vulnerable to Murphy's law. During her reciting of the coordinates Nanoha sneezes, and promptly vanishes. Yuuno realizing just how DEAD he is if Fate finds out, starts trying to figure out just where she went.
Meanwhile Nanoha wakes up in a very creepy castle to the sounds of combat and goes to check things out. What she wanders into is one of Larharl's fights to claim the title of overlord. His opponent, seeing a new figure on the battle field that isn't one of his, attacks her immediately. Nanoha's first reaction to what looks like, and actually is a demon attacking? Vaporize. Cue Larharl trying to recruit the scary powerful mage on the spot.
After the battle is over, with Nanoha and Larharl being the victors of course, they sit down and work out their temporary alliance. Of course such a conversation needs input from Etna, and Flonne joins in to make sure that Larharl doesn't take advantage of the sweet little girl. Nanoha just wants to go home but this proves to be more difficult than expected because of the swiftly escalating war between the Netherworld and Celestia, Larharl points out that if he becomes overlord he can end the war and send her home, so really it is in her best interest to help him out, right?
And so Nanoha sticks around and even gets a minion and a few prinnes to call her own, but eventually leaves for her home. She arrives back home to find a couple MONTHS have passed and everyone thought that she ether ran away or was kidnapped! So amidst all the smothering attention we move into the start of A's...