Martin Gibson, a famous science fiction author, is travelling to Mars, as a guest of the crew of the spaceship Ares. After arriving at Space Station One, in the orbit of Earth, from which all interplanetary journeys start, he makes the trip to Mars.
The youngest crew member, Jimmy Spencer, who is still in training to be an astronaut, is assigned the task of answering his questions about the technology of space flight, and they become friends. Gibson tells him about his early life, revealing that he had to leave Cambridge University because of a nervous breakdown and never completed his studies. After psychiatric treatment, he had become an author. He also reveals that he had an affair at university but that he and his girlfriend broke up and that she married another man, had a child and later died.
On Mars, Gibson and the crew go their separate ways. Gibson meets the Chief Executive of Mars, Warren Hadfield, and Mayor Whittaker, who run the colony from the base at Port Lowell. He discusses the future of the colony with Hadfield, who is keen to make Mars as self-sufficient as possible, given the vast distance that materials have to come from Earth.
On a trip by passenger jet to an outlying research station, Gibson and the crew are forced down by a dust storm. They explore the nearby area and discover a small group of kangaroo-like creatures, the unsuspected natives of Mars. They appear to have limited intelligence by human standards and are vegetarians, living on native plants.
It is later revealed that the plants are being cultivated by researchers to enrich the oxygen content of the Martian atmosphere. This project, and related others, are being kept secret from Earth.
Gibson discovers that Spencer is his son. In the meantime, Spencer has formed an attachment to Irene, Hadfield’s daughter.
Hadfield reveals that scientists have been working on "Project Dawn", which involves the ignition of the moon Phobos and its use as a second “sun” for Mars. It will burn for at least one thousand years and the extra heat, together with mass production of the oxygen-generating plants, will eventually – it is hoped – make the Martian atmosphere breathable for humans.
Gibson finds himself so persuaded of the importance of Mars as a self-sufficient world that he applies to stay on the planet, and is invited to take charge of public relations – in effect, to “sell” Mars to potential colonists.