It's the Second World War, and the Germans have just occupied the Channel Islands. The High Command have ordered a rescue mission on the (fictional) island of Amorel. David Niven is to lead the mission. He needs a local guide, and that turns out to be Glynis Johns. The preparations are extensive and unique, because they're trying to bring out a prize purebred cow.
Before you scoff, please be aware that something similar was done in rescuing the Aldenay breed from the Channel Island of that name. It's still absurd in retrospect, but everyone in the movie takes it seriously, which helps some with a cast that includes George Coulouris (in his first of nine appearances in director Ralph Thomas' movies) as the cultured cattle-breeding officer in charge of the island, and Noel Purcell as the navigator who knows the local waters. There's a suitable amount of flag-waving, islanders loyal to His Majesty, a cute kid, and a nasty German sergeant.
In short, this is pretty much a bog-standard movie with a great cast that makes it highly watchable. For me, I was enchanted as soon as Miss Johns' name appeared in the credits. There's extensive location shooting, with Sark standing in for the fictitious island, and a final race between the British PT boat and a pursuing German craft. This movie was near the end of the period when a serious movie could be made about such a subject. Soon enough, it would be all war-is-hell dramas and service comedies.