Roberta (also known as Bobbie), Peter, and Phyllis live comfortable, privileged lives with their loving mother and father in London. One day, everything changes: Father suddenly leaves the family home under mysterious circumstances and the children move to a modest house in the countryside to live a far more isolated and impoverished existence. The children do not understand why their father is absent, and their mother is reluctant to give them any details about his exact whereabouts.

The children cope with their adverse circumstances by forming a bond with their new home and community. They develop a deep love for the nearby railway, especially the train they nickname the Green Dragon. Each day, they wave at this train as it passes, and a passenger they nickname the old gentleman waves back to them. The children forge a friendship with the Porter, Mr. Perks.