John Carter is mysteriously conveyed to Mars, where he discovers two intelligent species continually embroiled in warfare. Although he is a prisoner of four-armed green men, his Civil War experience and Earth-trained musculature give him superior martial abilities, and he is treated with deference by this fierce race. Falling in love with a princess of red humanoids (two-armed but egg-bearing), he contrives a daring escape and later rescues the red men from the hostility of another nation of their own race. In this struggle he enlists the aid of his former captors, whom he gradually civilizes, teaching them first the practical advantages of kindness to their beasts of burden and then of casting aside centuries of communal living in favor of the nuclear family. At last he even starts them on the path to mastering the arts of friendship and diplomacy. When the failure of the atmosphere-generator threatens the planet's inhabitants with extinction, Carter's luck, memory, and sheer determination make possible the salvation of the planet, but Carter himself falls unconscious before he knows the success of his efforts. The novel ends with his sudden involuntary return to Earth.