A hundred years later, Katara and Sokka, two siblings who live in the Southern Water Tribe, free Aang and Appa from the iceberg. Katara is the only waterbender in her small tribe. She has taught herself how to bend. Her older brother, Sokka, cannot bend, but he is a warrior. Katara, Sokka, and Aang travel to the Northern Water Tribe so Aang and Katara can learn waterbending from a master. Aang does not know about the war or the genocide of the Air Nomads. While on their journey, they visit the Southern Air Temple, where Aang used to live. There, Aang discovers that the Fire Nation destroyed the Air Nomads. He also meets his Avatar guide, Avatar Roku, who is the Avatar who came before Aang. Roku tells Aang that he must learn to bend all four elements by the summer solstice. This is because Sozin's Comet will return on that day, and it will make the firebenders' bending very powerful. If Aang does not defeat the current Fire Lord, Ozai, by the summer solstice, then the Fire Nation will take over the world and no one will be able to stop them anymore.