Book One: Water contains 20 episodes.
Book One: Water is the first season of Avatar: The Last Airbender. It follows Aang after Katara and Sokka discover him in an iceberg and establishes the central quest: Aang must learn waterbending, earthbending, and firebending before confronting the Fire Nation.
The season aired on Nickelodeon from February 21 through December 2, 2005. Its 20 chapters take the group from the Southern Water Tribe through the Northern Water Tribe, while introducing major ideas such as the Avatar State, the spirit world, the Fire Nation’s century-long war, and the Moon and Ocean Spirits.
The season finale consists of “The Siege of the North,” presented as two episodes. That ending is sometimes involved in counting confusion because some broadcasts, listings, and home-video releases package episodes differently. The official season total remains 20, however. Book Two: Earth also has 20 episodes, while Book Three: Fire has 21, bringing the original series total to 61 episodes.