The Outer Hebridean island of Todday. The islanders lead uncomplicated but happy lives until, one day in 1943, tragedy strikes - no whisky. The news is too much for Captain McPhee, who falls ill and dies.
A gloom descends on the island. Postmaster Joseph McRoon has another worry - young schoolteacher George Campbell wants to marry his daughter, Catriona.
English officer Sgt. Odd, who is enamoured of Catriona's sister, Peggy, returns to Todday on the Island Queen after two years away. The skipper, Captain McKechnie, tells him the Home Guard's morale is low under its new English commander, Waggett.
Arriving at Todday, Sgt. Odd finds Joseph and a group of islanders at the quayside. He waves, but they ignore him, turning away dejected when they discover there is no whisky aboard. Waggett remonstrates with Captain McKechnie, who refuses to take on board some cases of ammunition which have been delivered in error. Odd suggests storing the cases at the Post Office.
George is confronted by his pious, domineering mother, who objects to his engagement. Later, Waggett oversees a Home Guard exercise. Waggett despairs at the men's lacklustre efforts at constructing a roadblock, and attracts the scorn of Dr. Maclaren, whose car is delayed.
At the beach, Odd proposes to Peggy. At her insistence, he asks her in faltering Gaelic and she accepts. Meanwhile Catriona berates George for not standing up to his mother.
The SS Cabinet Minister is lost in fog and runs into rocks off the coast. Sensing an opportunity for profit, two islanders, Biffer and Sammy, head out in a rowing boat. As he leads the crew ashore, he learns that the wrecked ship's cargo consisted of 50,000 cases of whisky. Biffer spreads the word around the island. Captain McKechnie takes the ship's crew to the mainland, and the islanders eagerly prepare their boats, until the chime of midnight signals the beginning of the Sabbath, and they gloomily turn back.
Waggett decides to guard the wreck, but his attempts to enlist George - his second-in-command - are frustrated by the formidable Mrs Campbell, who has locked George in his room. He finds Sgt. Odd with Joseph McRoon, where he is attempting to gain McRoon's consent to marry Peggy. McRoon leaves, and Waggett informs Odd of his plans, ordering Odd to take the first watch. After Waggett leaves. McRoon tells Odd that a wedding requires a rèiteach - a customary celebration - and a rèiteach must have whisky.
Midnight arrives and Sgt. Odd begins his watch. When Biffer, Sammy and another islander clumsily try to sneak up behind him, he allows himself to overpowered. Upon their signal, the boats head out. Sammy detours to collect George, who sneaks out through his bedroom window.
The islanders fill their boats with cases of whisky. But with Biffer still in the hold, the wreck begins to list. He is rescued by George, and they pull away just as the ship goes down.
As the islanders store the cases in a cave on the beach, Waggett finds Odd tied and bound. Waggett vows to find the perpetrators, and contacts his Colonel on the mainland. He is stunned to find the Colonel unsupportive. Unabashed, he contacts Constable McRae.
The island celebrates as the whisky flows again. Fortified by whisky, George takes on his mother.
Constable McRae's enquiries fail to uncover anything, but the innkeeper, Roderick MacRurie, angry at the loss of business, points Waggett in the direction of the cave.
Waggett lets it be known he is returning to the mainland with the unwanted ammunition. In his absence Peggy and Catriona's rèiteach begins. Waggett returns with Farquharson, an Excise Officer, and his men. They head for the Post Office, but the islanders have been forewarned, and the search uncovers nothing.
Waggett and Farquharson head for the cave, but the villagers set up a roadblock and beat them to the whisky. Waggett and Farquarson give chase, but are stopped by an impromptu roadblock.
Later, a dejected Waggett is summoned by customs to explain why one of the ammunition cases he sent to the mainland contained six bottles of contraband whisky. The islanders, and even his own wife, are overcome with mirth at his fate.
