The novel takes place sometime in the future, on the planet Mars. Humans have colonized the planet, but have not yet conquered its environment. As a result, the colonists must constantly protect themselves against external dangers—the harsh Martian winters, the oxygen-poor atmosphere, and the occasionally hostile Martian animals and plants. Heinlein frequently parallels the Martian environment with that of the American frontier, and the colonists with the pioneers who settled the American West.