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 (George Coulouris). He finds that the hereditary ruler, the Suzerain, is away in the British 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 due to the value of the cow's bloodline, and Major Morland (David Niven), is assigned the task of rescuing Venus. When he realises that the Suzerain's sister, Nicola Fallaize (Glynis Johns) is in Wales, serving as an Auxiliary Territorial Service army cook, she is quickly posted to the War Office and the two, with a radio operator sergeant and a Channel Islander naval officer who knows the local waters, are landed on the island.
They contact the Provost and discover that the Hauptmann, a cattle breeder in civilian life, is about to have the cow shipped to Germany. In a race against the Germans discovering their presence, they spirit the cow onto a beach and via a special craft, onto a Royal Navy Motor Torpedo Boat which takes them to Britain, though they are pursued by German E-boat.David Niven as Major Valentine Morland
Glynis Johns as Volunteer Nicola Fallaize
George Coulouris as Captain Weiss
Barry Jones as the Provost
Kenneth More as Lionel Fallaize, Nicola's cousin
Noel Purcell as 'Trawler' Langley
Bernard Lee as Brigadier
Jeremy Spenser as Georges
Patric Doonan as Sergeant Forbes
Martin Boddey as Sergeant Vogel
John Horsley as Naval Officer Kent
George Benson as Senior clerk
Richard Wattis as Carruthers
David Horne as Magistrate
Philip Stainton as Constable
Pat Nye as ATS auxiliary
The story is based on a real incident told to Tickell after the war by an army officer who was involved in a similar event.
The film follows the original novel closely with the exception of the fate of Lionel.
The fictitious island of Armorel may be based on Sark, one of the locations where the film was shot. Sark, inhabited by 500 people, had a feudal ruler, the Seigneur until 2008, as depicted in the play The Dame of Sark. Like all the other Channel Islands, it was occupied by German troops 1940–1945. British commandos made two unsuccessful raids in 1942–43.
The film was based on a novel. Much of the film was shot on the island of Sark. The island did not allow motorised traffic. The filmmakers were allowed one Land Rover and trailer to transport their equipment. Otherwise they had to walk or use boats and horse drawn carriages. The rushes were transported to the nearby island of Guernsey where they were seen weekly.
Director Ralph Thomas later said they used twelve plain coloured cows to play the lead cow, painting them with a patch on the side. He said this "was a sod because we shot mainly on location, and every time it rained, which it did regularly, the colours would run, and you would think the cow was milking itself because drops of paint were falling on the grass. It was a difficult picture but it was fun."