I just finished rereading this remarkable adventure story, and must say, some of the capsule summaries above leave a bit to be desired.
Yes, John Carter returns to Mars after 10 years absence, and finds himself in a paradise. But ironically, it's a paradise that soon turns into a kind of violent warring hell. The plant men are only the beginning. The bitterest enemies are a race of almost diabolical priests, the Tharns, who set loose white apes and plant men to slaughter thousands of religious pilgrims. I'm not spoiling the plot at all, since this becomes clear in the first two or three chapters.
Again, there's a deep bond between the hero, John Carter, and a brave and stunning young woman named Thuvia. John Carter, a warlord, is not content to merely escape. He must somehow end this evil empire of hypocritical priests and mass slaughter.
One fine element is the reintroduction of the green warrior chieftan, John's dear friend from the first novel in the series.
There are strains of deeper thought woven throughout. For example, the biting satire against mindless "fanatic" religions.
Heck of a book. Burroughs writes in a style that would seem a bit heavy today. But after a chapter or two, the reader usually can get in the stride. These books read fast-- you can't put them down. The heroines are GORGEOUS-- but the language is "clean" and the books can be recommended for teens. The description-- often in battle scenes-- is up to anything written in adventure science fiction today. This book was written-- amazingly-- in about 1917.