Issus is dead, but her high priest has activated a doomsday device that will destroy Mars's fragile atmosphere. Before they can shut it down, John Carter and his son Carthoris have to figure out where it is! Helping them is Linea, a beautiful black Martian princess with a very special heritage. Linea and Carthoris start to click, and a union between the black and red races could be just what the wounded planet needs. But unknown horrors await the young lovers at the end of their quest.