Hoping to cut off the threat at the root, he travels undercover to the Assassins' base, the restive city of Zodanga, still smarting from its defeat and sack by the Empire of Helium and the horde of Tharks in A Princess of Mars.

Complications ensue as the two threads of the plot become entangled, and the Guild, in its own attempt at a preemptive strike against Carter, kidnaps his wife, the princess Dejah Thoris of Helium.

For the first time the action of the series goes off-planet, as Ur Jan and Gar Nal flee with Dejah to the Martian moon Thuria (Phobos) in one of the spacecraft, pursued by Carter and his allies Jat Or and Zanda in the other.

Amid betrayal and heroic sacrifice, the parties from Mars eventually return to their home planet with Carter and Dejah still separated and the latter believed still captive on Thuria.

The first letter of the preface and the subsequent twenty-four chapters of the book form an acrostic message from the author to his second wife Florence, whom he was in the process of marrying at the time of publication.