Roberta, Peter and Phyllis have a perfect life, until their 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, they 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. Then they befriend an Old Gentleman who regularly takes the 9:15 train near their home. He is eventually able to help to prove their father's innocence, and the family is reunited.