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...