At the end of A PRINCESS OF MARS, the first volume in Burroughs's Mars series, John Carter managed to get the factory that produces oxygen for Barsoom working again -- and collapsed. When he came to, he found himself back on earth, and separated from his beloved Dejah Thoris, the Princess of Helium. It's a decade later when Carter returns to Barsoom, and he finds himself in that part of the planet that the natives consider to be "heaven" -- which is no heaven at all. Carter has to reunite with his friend the fierce green warrior Tars Tarkas, fight with plant men and the great white apes of Barsoom, violate some significant religious taboos, survive the affections of an evil goddess, foment a slave revolt, fight in an arena, and still save Dejah Thoris in the middle of a giant air battle between the red, green, black and white people of Barsoom. High adventure, Martian style.
As I stood upon the bluff before my cottage on that clear cold night in the early part of March, 1886, the noble Hudson flowing like the grey and silent spectre of a dead river below me, I felt again the strange, compelling influence of the mighty god of war, my beloved Mars, which for ten long and lonesome years I had implored with outstretched arms to carry me back to my lost love. Not since that other March night in 1866, when I had stood without that Arizona cave in which my still and lifeless body lay wrapped in the similitude of earthly death had I felt the irresistible attraction of the god of my profession. With arms outstretched toward the red eye of the great star I stood praying for a return of that strange power which twice had drawn me through the immensity of space, praying as I had prayed on a thousand nights before during the long ten years that I had waited and hoped.