When we last saw John Carter, he had been accidentally transferred back to Earth, and away from his beloved Dejah Thoris. 
 Well, obviously in a novel named "The Gods of Mars," he isn't going to stay on his home planet and learn how to play poker. Instead, Edgar Rice Burroughs' second Barsoom novel returns our hero to the red planet, and introduces a whole new dilemma for our hero -- how to get back to the world of the living. Yes, he's both alive AND dead, as far as the Barsoomians are concerned. 
 John Carter returns to Mars, but is shocked to find that he's in a completely unfamiliar part of it, populated by grotesque Plant Men and white apes. After encountering his old friend Tars Tarkas, Carter learns that they are in the Barsoomian afterlife. No, really. The problem is, nobody can return from the pilgrimage to the River Iss -- and if anyone tries, they will be killed. Even though they were already dead... don't think too hard about it. 
 But the greatest threat comes from two breeds of Martian that Carter has never seen before -- the Black Martian pirates and the White Martian Therns, both of whom consider themselves to be gods. Anyone who survives the valley is turned into a slave, and Carter soon makes new allies among his fellow captives. But even if he can escape the grasp of the "goddess" Issus, he may not be able to survive in a world where escaping from "death" is blasphemy -- especially when he finds that Dejah Thoris may be doomed as well. 
 "The Gods of Mars" is a much faster-moving story than the first Barsoom book, even though we're introduced to a whole new part of Mars and two new Martian species. Rather than the slow introductions of "A Princess of Mars," Burroughs just plunges right into the main story right away, and it's a long time before he stops to contemplate the flora/fauna/customs of the Martians. 
 Burroughs' prose is also more polished in this story, with longer action scenes and a faster pace -- as well as some moments that are absolutely horrifying (the plant men, with their corpselike skin, ragged noses and wormy hair). And while he still has the slight stuffiness of early 20th-century pulp, he injects a lot of vivid descriptions and action into the story. 
 John Carter is still a bit on the Stuey side -- multiple women are in love with him by the story's end, for instance. But he's still a very strong character, with lots of guts and courage. Burroughs throws a rather unexpected twist into the story when Carter discovers that one of his fellow captives is actually related to him; and he also introduces some other interesting characters, such as the malignant Issus and the strong, proud Thuvia. 
 "The Gods of Mars" is one of the few sequels that is actually markedly better than the original book. But be warned: it leaves you dangling on a cliffhanger that will leave you scrabbling for the third book.