 The Germans invade the British-owned Channel island of Amorel on July 10, 1940, and though strategically unimportant, the occupation causes great consternation to the Ministry of Agriculture. The small island is the home of "Venus", a prize cow "with a pedigree as noble as the Kings of Ireland - a triumph of breeding." Plus, "Venus" is in calf - by "Mars of Mellowbury III", a champion bull - and the crossing of "Venus" and "Mars" is of enormous financial and breeding potential, horoscopes aside. The Ministry approaches the War Office, where "Venus" becomes the concern of the Armed Forces of the Crown. Major Valentine Moreland (David Niven) and his crew, A.T.S. Nicola Fallaize (Glynis Johns), radio operator Forbes (Patric Doonan) and Trawler Langley (Noel Purcell), are assigned the task of quietly re-invading Amorel and bringing "Venus" to England. The simple plan of keeping the cow in hiding until a British destroyer arrives to pick her up is complicated when Commander Weiss (George Coulouris), the German garrison commander, learns of the cow s value, and intends to send her to Germany in an E-Boat which is scheduled to be in the harbour the same day as the British ship.