The Clouded Yellow (1951)

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.