After ten years on Earth, our hero, Barsoom's most beloved hero and saviour the great and powerful almighty John Carter, finally figures out the 'secret' to interplanetary travel that he used all those years ago.
He
just
thinks of Mars or something. It's not explained. And boom! He's in a lush dense forest!
Wait. This isn't Mars, it's can't be bone dry with only the trickle stream the Great Iss as any important amount of water....
And what are those strange... things... coming my way... like with a giant hole for a nose and mouths for fingers and ears at the end of their hair ... and I think eyes in the middle of their heads and I think they look like leeks, personally the way he describes them. Giant carnivorous leeks. Oh and for nostalgia's sake, giant six-limbed white apes.
These are the fearful truth that awaits any Barsoomian who takes the voluntary "Final Pilgrimage" down the Iss, as Barsoomians either die in battle, or at the age of some thousand years they take the long pilgrimage down the Iss "never to return".
It's supposed to be the Martian Heaven, an Elysian field ruled by a benevolent goddess, but as Carter soon learns, this final naturally irrigated area of the dying planet is home to nested levels of cannibalous beings, who keep alive their "lessers" so that they may harvest them for either slaves or food.
He manages to find old friends and make new ones, even amongst the First Born, the most ancient sapient race of all of Barsoom, who hole up at the south pole with their goddess and eat the rest of the Barsoomians.
He meets his son - yes despite all Barsoomians being oviparous and about ten times the lifespan, they are sexually compatible with Earth humans.
(That wasn't a surprise at all you could see it coming ten miles away. "Oh who oh who could this boy be?". Oh, and somehow this is ten (Earth) years later and the boy is basically a young adult, trying for an Odysseus/Telemachus theme.

And the Brilliant Saviour of all Marskind John Carter, Jedak of Helium is in his sworn duty to protect all of Barsoom from the hideous evil of this false religion!
The amazing 1% atmosphere of Mars really helps his fighting ability. Attacked by man-eating plants and giant gorillas, then dozens or random fierce animals, with absolutely no rest at all, then on to another fight... He is basically always on the go, except when he does get a chance to sleep (I think 5 times in the book) he always gets rudely awakened.
But magic Mars Atmosphere Carter (perhaps it's the proximity to the home sun like the Kryptonians?), is even kidnapped for a whole 365 days, trapped in an underground cell, with just a bit of food and maybe water every day, no light at all except from the torch of the guy who brought the food, unconversationally, for 365 days, his leg in irons, and TWO days of travel later he takes on literally legendary armies. Literally harrows out the Martian hell. If you needed a bigger Jesus cliché.

Loved that crazy revolving torture tower idea tho. Really cool.
But. If they were tossing canned food into the ever decreasing hole (Just really imaging the scale of this thing to be moving at the strange speed it is (0º30'/d) :::: knowing the diet of the locals, what is that food made of, and who canned it, how and why? I guess it's okay because it's food enough? How big is that cell, I think it would definitely cause insanity... Is it stark bright white? That's how I imagined it. Would 100% need ventilation to work.
[[Edit - Did the math - Even if the chambers were 2 kilomteres wide, it would at the mouth be moving at 3m/h (that's metres to you metricophobes). That they could jump out of that "swiftly" moving ride preeety easily, unless I'm missing something very important...