"We're returning to earth," Paul's father tells him. Paul is wildly excited, for all human beings on the planet Ganymede dream of going back to Earth some day. Then Paul finds out that he cannot take his robot Rex with him. Rex has been his constant companion for sixteen years. Leave him behind? Never!So begins a series of breathtaking adventures in space as Paul and his robot Rex attempt to outwit the forces that seek to separate them.