This is the tale of one Vor Daj. John Carter went in search of Ras Tavas, and yielding to pleas, brought along one soldier. They quickly find that finding him will not be easy.
Indeed, they are taken prisoner, along with some others, include one woman, and taken to the city, encircled by marshes, where they find that Ras Thavas is the prisoner of his own synthetic men, and forced to produce more and more of the virtually unkillable monsters. And to transplant the brains of the most powerful of them to the bodies of red men -- much to the peril of Janai, the woman.
Ras Thavas is perfectly willing to do what John Carter asks -- if they can only escape. And Vor Daj is worried about Janai's fate.
The rest of the tale involves disappearance, claims to the throne, bodyguards, a cowardly race that regards sea shells as treasure, an experiment spreading wildly, and much more