Sea devils (1953)

Gilliatt, a fisherman-turned-smuggler on the isle of Guernsey, agrees to transport a beautiful woman to the French coast in the year 1800. She tells him she hopes to rescue her brother from the guillotine. Gilliatt finds himself falling in love and so feels betrayed when he later learns this woman is a countess helping Napoleon plan an invasion of England. In reality, however, the "countess" is an English agent working to thwart this invasion. When Gilliatt finds this out, he returns to France to rescue the woman who's true purpose has been discovered by the French.