Dejah Thoris, Princess of Helium and wife of John Carter, is captured and imprisoned in the Temple of the Sun, in a revolving room whose entrance is only accessible once a year. She shares her cell with Thuvia of Ptarth, another Barsoomian princess, as well as Phaidor, the daughter of a priest. The three are rescued by antagonistic figures, one of whom decides to take Thoris in order to spite Carter. Carter pursues them, bent on rescuing his wife.
Like the preceding novels, The Warlord of Mars is a science fiction novel, under the sub-genre of planetary romance (with some sword and sorcery thrown in for good measure). Some spacecraft do appear (a hallmark of a space opera), but their importance is not central to the story.