The story concerns a family who move from London to "The Three Chimneys", a house near the railway in Yorkshire, after the father, who works at the Foreign Office, is imprisoned after being falsely accused of spying. The children befriend an Old Gentleman who regularly takes the 9:15 train near their home; he is eventually able to help prove their father's innocence, and the family is reunited. The family takes care of a Russian exile, Mr Szczepansky, who came to England looking for his family (later located) and Jim, the grandson of the Old Gentleman, who suffers a broken leg in a tunnel.