Tom Billings is the best friend of Bowen Tyler. When he gets his message from the last book he mounts a rescue expedition. They find Caspak and Tom flies over the cliffs in a seaplane that he brought along. It is immediately attacked by a pterodactyl and he crash lands.
He rescues a beautiful woman named Ajor from a band of ape men. The two set out to return her to her people the Galus(rope men) farther up the river. They pass through the Bo-lu(club men),Sto-lu(hatchet men),Band-lu(spear men), and the Kro-lu(bow men). Each represent a more advanced human in the scale of evolution.
It is later discovered that life in Caspak goes through stages of metamorphosis after a period of time. Tom and Ajor eventually reach the fully human land of the Galus. There he meets his friend Bowen and the crew from his rescue expedition who scaled the cliffs.
The second in the trilogy is another excellent book. We get to find out the secret of the evolutionary phenomenon of Caspak. There is still one more book to tie up loose ends which will be discussed later.
The third Burroughs movie made in the seventies combined elements from the second and third book. It starred Patrick Wayne, John Wayne’s son and saw the return of Doug McClure. I think this is the best of the three movies.