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This story was originally published as one half of an Ace Double novel, and therefore is rather short; it also is a sequel to an earlier novel, "The Coils of Time". The beginning of this novel summarizes the previous one, so you don't have to read it to understand this one. The previous story had the hero (Christopher Wilkinson) lose his fiancé to an accident, so he seeks out a means of time travel, to change his beloved's death; instead of time machine, though, he discovers a machine that can travel between alternate universes, and in another universe, he finds his fiancé again, and they end up returning to the heroes' original universe.
Now, the scientist who helped the Wilkinson create the alternate-universe traveling machine has a theory that popular stories about Mars (particularly "War of the Worlds" (by H.G. Wells) and the Barsoom novels (By E.R. Burroughs)) are really memories from alternate universes. Since the scientist has always wanted to see Barsoom, he decides to travel to Mars, and test out his machine to see if he can reach the fabled place.
The heroes then travel to Mars in a space ship that Wilkinson has become the captain of, and test out their machine. This part of the novel takes 60 pages (out of a total of 120), and is very boring. I'm not sure why the author thought he should take so long to get going, because absolutely nothing happens, and there aren't even any good philosophical discussions among characters; it is merely a description about handling the spaceship, the meals they eat, and the bureaucracy that they have to navigate through in order to land on Mars legally.
Anyway, the machine works, and they start to explore the alternate Mars, which is soon proved to be a conglomeration of both the evil Martians from "The War of the Worlds" and the Tharks from "A Princess of Mars", but where human beings are slaves, and considered less than intelligent.
The idea for this story was a good one, but unfortunately by the time that the main theme is arrived at, there is only 60 pages to explore that alternate Mars, and it ends up being a pretty standard science fiction story. I won't say how it ends, but there is not a single surprise.