Adrien is the crown prince of the fire nation which at the time lives happily in peace with the water tribe, air temple, and earth kingdom. Until Adrien is discovered to be the avatar and his father plans to use his gift so the fire nation can rain supreme. But Adrien having been raised by his mother that the four lands are meant to live together equally refuses. So his father decides to force him to do it. Not wanting his gift to be used for evil, Adrien runs away from home with his pet dragon Plagg and vanishes.
Eighteen years have passed and the world is in a terrible war with the fire nation ruling supreme. Marinette is the last water bender off her tribe after the fire nation in prisoned her people including her parents when she was a child. She managed to avoid this fate after her mother placed her on life boat that was the enemy ship's along with her polar bear dog Tikki. Her boat drifts to the air temple where she's adopted by a family who's son Nino is an air bender.
When she's thirteen, she and Tikki discover a drifting iceberg which has a boy and animal in it. She frees them and they turn out to be Adrien and Plagg, who is instantly smitten with the young girl. He learns from her that his father has caused a war and decides to train to stop his father. So he, Marinette, Nino, and their faithful pets Plagg, Tikki, and Wayzz: Nino's flying bison start their journey. Little do they know that Felix, Adrien's "younger brother" who was a baby when Adrien ran away has been assigned by his father to capture the avatar. (Gabriel thinks it's a new avatar, he thinks Adrien is dead and he never told Felix that he had a brother)