After leaving the British Secret Service, David Somers (played by Trevor Howard) finds work cataloging butterflies at the country house of Nicholas and Jess Fenton. After the murder of a local gamekeeper, suspicion (wrongfully) falls on their niece, Sophie Malraux (Jean Simmons). Somers helps Sophie to escape arrest and they go on the run together. After a cross-country chase they arrive at a coastal city with the intention of leaving the country by ship. All's well that ends well after the true identity of the murderer is revealed.