Bradley an Englishman from the captured U-boat was leading an expedition to explore Caspak. While standing guard one night he is captured by strange men with bat wings. He is taken to an island in the inland sea. There in the city of skulls he finds himself prisoner of the Weiroo in the land of Oo-oh.
The Weiroo are the most advanced race in Caspak but somewhat of a dead end in the evolutionary scheme of things. The Weiroo are all male and must capture human females from the mainland to mate. The offspring are always male so they are always trying to find the secret to producing a female Weiroo.
Bradley meets Co-tan a female captive from the mainland. Together they escape the city and are able to capture two Weiroo. They force them to fly them to the mainland. There Bradley reunites with Bowen, Tom Billings and the crews of the U-boat and rescue expedition. All the survivors leave Caspak with the heroes marrying their respective loves.
The final book in the Caspak trilogy. It was probably the most interesting book. I thought that Caspak was a unique and exciting creation of Burroughs. The story was tied up neatly at the end. I was familiar with the stories from the Doug McClure movies and was not disappointed with the books.