John Carter, an American Civil War veteran, goes prospecting in Arizona and, when set upon by Indians, is mysteriously transported to Mars, called "Barsoom" by its inhabitants. Carter finds that he has great strength on this planet, due to its lesser gravity. Carter soon falls in among the Tharks, a nomadic tribe of the planet's warlike, four-armed, green inhabitants. Thanks to his strength and combat abilities he rises in position in the tribe and earns the respect eventually the friendship of Tars Tarkas, one of the Thark chiefs.
The Tharks subsequently capture Dejah Thoris, Princess of Helium, a member of the humanoid red Martian race. The red Martians inhabit a loose network of city states and control the desert planet's canals, along which its agriculture is concentrated. Carter rescues her from the green men to return her to her people. (Summary from Wikipedia)