Summary:

Arianna is overshadowed by her identical twin, Melanie. On the day her little brother is born, Ari and 20 other girls are kidnapped by her Water Tribe grandfather.
Eight years later, Ari has grown into a independent teenager who wants revenge on her grandfather, the King of the Water Tribe, for kidnapping her and forcing them to be slaves.
When they escape, thanks to a kind-hearted advisor of the King, they travel home and survive a invasion from the Water Tribe. Ari's family is missing and she has to figure out the new duties of the Avatar and how she'll find them and elude the Water Tribe.