A pair of identical twins, Allen and George Carter, have been separated at birth as part of a twin study to settle the nature versus nurture question. Allen is raised on a highly developed Earth, while George has been raised in the provincial frontier society of Ganymede. On their twenty-fifth birthday, they are introduced to each other and given the task of running the family farm on Mars, where they have to work together in both day-to-day matters and unusual events.