The story is set in a future, overcrowded Earth, where food is carefully rationed. Teenager William (Bill) Lermer lives with his widower father, George. George decides to emigrate to the farming colony on Ganymede, one of Jupiter's moons. After marrying Molly Kenyon, George embarks with Bill and Molly's daughter Peggy on the 'torchship' Mayflower . On the journey, Bill saves his bunkmates from asphyxiation by improvising a patch when a meteor punctures their compartment. During the trip all the children attend class; also, to combat the boredom of the long trip, the Boy Scouts among the passengers form troops.
When they arrive on Ganymede, an unpleasant surprise awaits the newcomers. The group is much larger than the colony can easily absorb and the farms they were promised do not yet exist. In fact, the soil has to be created from scratch by pulverizing boulders and lava flows, and seeding the resulting dust with carefully formulated organic material. While some whine about the injustice of it all, Bill accepts an invitation to live with a prosperous farmer and his family to learn what he needs to know, while his father signs on as an engineer in town. Peggy is unable to adjust to the low pressure atmosphere and has to stay in a bubble in the hospital. When the Lermers are finally reunited on their own homestead, they build their house with a pressurized room for Peggy.
One day, a rare alignment of all of Jupiter's major moons causes a devastating moon quake which damages most of the buildings. Peggy is seriously injured when her room suffers an explosive decompression. Even worse, the machinery that maintains Ganymede's "heat trap" is knocked out and the temperature starts dropping rapidly. George quickly realizes what has happened and gets his family to the safety of the town. Others do not grasp their peril soon enough and either stay in their homes or start for town too late; two-thirds of the colonists perish, either from the quake or by freezing. The Lermers consider returning to Earth, but after Peggy dies and in true pioneer spirit, they decide to stay and rebuild.
The colony gradually recovers and an expedition is organized to survey more of Ganymede. Bill goes along as the cook. While exploring, he and a friend discover artifacts of an alien civilization, including a working land vehicle that has legs, like a large metal centipede. This proves fortuitous when Bill's appendix bursts and they miss the rendezvous. The shuttle picks up the rest of the group and leaves without the pair. They travel cross country to reach the next landing site. Bill is then taken to the hospital for a life-saving operation.