Thuvia Maid of Mars by Edgar Rice Burroughs

Carthoris is madly in love with Thuvia. This love was foreshadowed at the end of the previous novel. Unfortunately Thuvia is promised to Kulan Tith, Jeddak of Kaol. On Barsoom nothing can break an engagement between a man and woman except death, although the new suitor may not cause that death. Thus it is that Thuvia will have none of him. This situation leaves Carthoris in a predicament. As Thuvia suffers the common Burroughsian heroine's fate of being kidnapped and in need of rescue, Carthoris' goal is abetted by circumstances. Thus he sets out to find the love of his life. His craft is sabotaged and he finds himself deep in the undiscovered south of Barsoom, in the ruins of ancient Aanthor. Thuvia's kidnappers, the Dusar, have taken her there as well, and Carthoris is just in time to spot Thuvia and her kidnappers under assault by a green man of the hordes of Torquas. Carthoris leaps to her rescue in the style of his father.