After the events of A Princess of Mars, John Carter returned to Earth and spent ten long years trying to find a way back. In our world, Edgar Rice Burroughs produced the sequel, The Gods of Mars, in 1913, a year after Under the Moons of Mars was serialized. Like its predecessor (and Tarzan of the Apes, which he published in 1912 also), The Gods of Mars was serialized in the All-Story Magazine incarnation of Argosy. It would be published in novel form in 1918.
John Carter returns to Mars in a way similar to how he first got there: by astral projection/wishing really hard. Again, the “How” he gets there isn't important, its just a means to an end. He arrives on Barsoom the same way as the first time: bare-ass naked.
Only this time, he's in a scenic forest in a river valley.
Populated by white apes and carnivorous tree-men that want to kill and eat anyone who arrives.
Welcome to Martian Heaven: The Valley Dor, endpoint of the sacred River Iss. Leaving the valley is considered blasphemy and cause for execution.
Martian Heaven sucks.
On arriving, John sees some Green Martians attacked and killed by the local fauna, and rushes to the aid of the lone survivor, who happens to be Tars Tarkas, Jeddak of Thark and Carter's best friend on Mars not named Woola.
The two fight their way out of the forest and into the cliffs of the valley, where they find the Holy Therns, white skinned Martians who claim to be the gods of Mars and control the creatures of the valley, enslaving anyone who survives them. They also wear cute little blonde Prince Valiant wigs because they're bald and vain.
John kills one of their priests, takes his wig and costume as a disguise, rescues a Red Martian woman named Thuvia, and the three rampage their way through the Thern fortress, reaching the top only to find it attacked by sky pirates. John sends Thuvia and Tars Tarkas off and boards one of their airships, killing the crew save one, Xodar, a prince of the Black Martians, the self-proclaimed First Born of Mars, and incidentally, a people also claiming to be gods. He also rescues a White Martian girl, Phaidor, who falls in love with John, but John is faithful to his wife Dejah Thoris, and is also disturbed by Phaidor's casual cruelty.
Xodar has some tricks up his sleeve, and John & Phaidor are captured by reinforcements, and taken to Omean, an underground sea where the Black Martians make their home. The First Born are ruled by the cruel (self-proclaimed again) goddess Issus, and like the Therns, they enslave the other Martian races to serve them and fight in their arenas.
Hmm: long-lived, beautifully-featured, ebony-skinned, underground raiders who worship a cruel goddess and love to party at the expense of others.
It really activates the almonds.
Anyway, Phaidor is taken to be a handmaiden of Issus. John is sent to the arena. Xodar is sent as well, since he lost in combat to John, and must serve him. Naturally, the two become best buds working to escape from their virtual death sentence. In prison, they find a pale young Red Martian youth who is obviously John Carter's son Carthoris, but the book plays coy with it for a while until the two are able to have a moment's rest.
There's already so much going on that going through the plot would take pages and pages. John Carter escapes. We get back to Helium and Red Martian intrigue. A reunion with cool dude Kantos Kan. Dejah Thoris went to the Valley Dor to look for her husband and son. John Carter wants to go after her. More intrigue. A titanic air battle between the fleets of three navies, and John Carter cutting down any bastard that stands between him and Dejah Thoris.
And then it ends on a massive cliffhanger because by this point Burroughs knew he had his audience hooked.
While some of the twists are blatantly telegraphed and a lot of convenient coincidences take place that stretch belief, the book hits the ground running and never lets up on the adventure.
The section that stands out the most to me is where John is taken to the arena and sees a group of female slaves inspected by the hideous Issus and a few are selected and taken away. John learns that the ones taken away were to be eaten by Issus and her court, while the rest would be torn apart by animals in the arena.
In a modern story, John would feel bad as he watched the innocent women get slaughtered for the entertainment of the First Born, then do some arena fighting, then plot his escape, then try and fail to escape, then mope a lot, then try again, then maybe succeed and that would take up half of the book and whoever was helping him escape would die and he'd feel extra bad about it.
Not here. This is Pulp! As soon as John learns that the women are going to be killed, the red mist descends over his eyes and he punches his way out of his cage (because Earthmen have super strength on Barsoom, remember?) and begins butchering everything between him and the throne of Issus. Not only does Carthoris join him in this, but every single slave in the arena, man and woman, join his uprising and John Carter nearly succeeds in reaching Issus, if not for the ancient hag queen's trickery. And this is only about halfway into the book!
Burroughs is awesome. John Carter is awesome. The Gods of Mars is awesome. Read the Barsoom books. That's an order.