Ulysses Paxton, fighting in World War I, finds himself transported to Mars with no more reason than John Carter -- though this goes more briskly than in Princess. He finds himself in the lair of Ras Thavas, a slightly mad scientist who transplants organs, including brains, and can often revive the dead. He has room after room of bodies suspended as if time did not pass.
He trains Paxton, thinking that a man with nowhere to go is the most trustworthy he can find, and knowing he needs someone to transplant his own.
But Paxton finds him aiding both the injured and the corrupt -- in particular, selling the body of a stunning young woman to a ugly old jeddaka -- in stunning indifference to cruelty, kindness, money, and anything else but knowledge. When Paxton revives the ugly old jeddaka with the young woman's mind, she can cope with the change, observing that her beauty had its upside and its downside, and even refuses transplantation into a body that is younger and healther, because Ras Thavas might sell it. But when Paxton demands, as a price of operating on him, that he restore her if Paxton brings back her original body, he finds that Ras Thavas ordered her death, and he restores her to suspension.
But he has his task before him. The rest of the tale involves launching an unmanned flyer, a young man assassinated so that a court favorite could woo his love, a jeddak's popular nephew, an apparent white ape that can talk and read, an empty idol, a promise technically fulfilled, someone who aids him while believing his plan mad, and much more.