On 20 July 1969, Apollo 11 astronauts Neil Armstrong and Joe Muldoon walk on the Moon, and Nixon's "most historic phone call" is joined by a call from former President Kennedy, committing the United States to send a crewed mission to Mars, which Nixon backs as part of his fateful decision to decide the future of crewed spaceflight, instead of deciding on the Space Shuttle program as he did in our timeline.

Preparations for this new goal include slashing the number of Moon landings so funding and leftover Apollo spacecraft hardware can go towards the efforts of the crewed Mars mission.

In the aftermath, a new Mars mission plan dubbed Ares is drawn up, utilising the upgraded Saturn V-B, which has numerous improvements, including the use of solid rocket boosters to double its payload.

Ares also uses on-orbit assembly of a different long duration Mars-ship using wet-workshop Saturn rocket components as the propulsion systems as well as a Skylab habitat module and external tanks to hold extra fuel.

The allohistorical setting of the novel was further explored in the short story "Prospero One", in which Baxter focuses on alternate developments in the 1960s British space programme – namely, its first and only crewed flight.