This classic story opens with a classic entrance, as Katara tells us all about the world of Avatar. A land of nations, four to be precise. Each governed and defined by their element. Water, Earth, Fire, and Air.
Keeping the peace was a being called the Avatar, who, unlike all others, could control all four elements. But when the world needed him most, in an event known as the Hundred Year War, the Avatar was nowhere to be found.
We arrive in the present, and find that the world has been ravaged by the Fire Nation. Though they have not conquered the world, they have control over most of it, and have oppressed much of the rest. This includes the South Pole, and the small tribe that lives there. The village is full of women and young children, which include Katara and Sokka.
During a fishing trip, they begin to argue, and accidentally uncover a giant ball of ice, that just so happens to contain a young boy inside. This boy breaks apart the ice, and reveals himself to be Aang once awake, he is not alone though, for he has his trusty Air Bison Appa with him as well.
Katara and Sokka are taken aback by this, for an Air Nomad (as they are called) has not been seen in a long time. Furthermore, the Avatar (in the current place in the cycle) was supposed to be an Airbender. They question Aang about it, but he doesn't give a definite answer.
After taking him back to their village, Aang begins to realize the gravity of the situation he is in. He finds out that the Air Nomads are "extinct", and begins to wonder what he should do. However, as he is still a child, he decides to sidestep, and focus on having fun. This leads him and Katara to a defunct Fire Nation ship. Aang and Katara explore the ship, and accidentally set off a flare.
Far away, a disgraced Fire Nation Prince names Zuko sees the flare, and sets course to where it went off. He feels his saving grace has finally come.