When Roberta, Peter and Phyllis's father is imprisoned after being falsely accused of spying, they and their mother have to leave their comfortable London home to go and live in a small house in the country.

However, the children soon come to love the railway that runs near their cottage, and have many adventures - stopping a train from disaster, saving an infant and dog from a barge canal on fire, and getting help when they find an injured boy in the trail tunnel.

When they befriend an Old Gentleman who regularly takes the 9:15 train near their home, he helps them to prove their father's innocence, and the family is reunited at last.