At the end of the previous book, John Carter's wife, the princess Dejah Thoris, is imprisoned in the Temple of the Sun by the vile pretender goddess Issus.

In the meantime, Carter's friend Xodar has become the new Jeddak (chief or king) of the black Martian First Born, and those white Martian therns who reject the old religion likewise gain a new unnamed leader, but there are still some who wish to keep the old discredited religion going, including the therns' erstwhile leader, the Holy Hekkador Matai Shang.

In the equatorial Land of Kaol, on the opposite side of the planet from Helium, their jeddak Kulan Tith has not yet abandoned the old religion, and accepted Matai Shang's request for safe haven.

Kulan Tith orders Matai Shang to deliver Dejah Thoris and Thuvia, but instead, he and Thurid take the women and flee to the north.

After traversing through the Carrion Caves which cross the barrier, Carter and Thuvan Dihn encounter Talu, the rebellious nephew of the tyrant Salensus Oll, who rules the yellow Martians' realm of Okar from the city of Kadabra.

Carter rescues Tardos Mors and Mors Kajak of Helium, triggering a rebellion among the Heliumite prisoners, which turns into a full-fledged invasion after Carter deactivates the yellow Martians' magnetic tower that wrecks invading fleets so Helium's fleet can land safely, carrying a volunteer force of Green Martians headed by Tars Tarkas as well.

[8] Most of the action in a planetary romance is on the surface of an alien world, usually includes sword fighting, monsters, supernatural elements as telepathy rather than magic, and involves civilizations echoing those on Earth in pre-technological eras, particularly composed of kingdoms or theocratic nations.

[7] Burroughs' vision of Mars was loosely inspired by astronomical speculation of the time, especially that of Percival Lowell, who saw the planet as a formerly Earthlike world now becoming less hospitable to life due to its advanced age, whose inhabitants had built canals to bring water from the polar caps to irrigate the remaining arable land.

[13] The Warlord of Mars introduces the Yellow Martians, supposedly extinct, whom John Carter finds in secret domed cities at the north pole.

They are black-bearded, exceptionally cruel, and keep slaves, acquiring these by using a giant magnetic device which sends fliers off course, and allows the Yellow Martians to capture the occupants.