In 1940, after the fall of France, the fictitious Channel Island of Armorel is occupied by a small garrison of German troops under the benign command of Hauptmann Weiss. He finds that the hereditary ruler, the Suzerain, is away in the army, leaving the Provost in charge.Back in London, the Ministry of Agriculture realise that Venus, a valuable pedigree Guernsey cow, remains on the island. They petition the War Office to mount a rescue operation, and Major Valentine Morland is assigned the mission, with the assistance of the Suzerain's sister Nicola Fallaize who joined the A.T.S. at the outbreak of war.They travel to Armorel by submarine, contact the Provost and other friends on the island, and discover that Weiss, a cattle breeder in civilian life, is about to have the cow shipped to Germany. By a series of elaborate deceptions, they extract Venus from Weiss's command and succeed in returning her to England.