It has been 10 years since John Carter was transported back to Earth from the last book. He finally figures out how to transport himself back to Mars. Unfortunately he ends up on the one place that no one is ever allowed to leave. The Valley Dor is the Barsoomian equivalent to heaven and were all Barsoomians make a pilgrimage when they get tired of life. Once there the penalty is death for anyone that attempts to leave.
Carter finds out soon that Dor is anything but heaven. The valley is full of vicious plant men and white apes that ambush and eat the unsuspecting pilgrims. The Therns a race of whites with blond wigs run the place. Carter saves his friend Tars Tarkas from the creatures. Together with him and a red slave girl named Thuvia, the trio escape the valley.
Carter is captured by the black pirates of Barsoom on one of their raids on the Therns. He is taken back to their secret city in the south pole. There he finds them ruled by an old hag that claims to be the goddess Issus. Issus has her slaves put to death after one year. Carter decides to escape and joins forces with a disgraced black pirate named Xodar and a mysterious red youth. The youth later turns out to be his son Carthoris.
Carter and his companions escape back to Helium. Here they face another enemy. Dejah Thoris has taken the pilgrimage to Dor. Her father and grandfather have been missing searching for Carter and Carthoris. The Zodangan jed Zat Arras is in charge and wants revenge for Carter’s sacking of Zodanga. He has Carter tried for heresy and sentenced to death.
Carter is forced to flee with his loyal followers and assault the Therns and pirates in a climactic confrontation with Zat Arras following behind. He is too late to rescue Dejah. Issus has her and two other women that love Carter, Thuvia and a Thern named Phaidor imprisoned in a chamber that won’t open for a year.
This has got to be Burroughs finest work. There is a lot going on. Carter goes from one adventure to another and hardly lets the reader catch his breath. The exotic locations, characters and beasts just enhance the story. It ends with a cliffhanger that demands a sequel.