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 during the evacuation of the island, Venus, a prize pedigree cow, has been left behind. They petition the War Office to do something urgently, and Major Morland is assigned the task of rescuing Venus. When he realises that the Suzerain's sister, Nicola Fallaize is in Wales, serving as an army cook, she is quickly posted to the War Office and the two, with a sergeant and a naval officer, are landed on the island...