During the Second World War, newly promoted Major Stringer [José Ferrer] of the Royal Marines conceives a bold plan to strike at an enemy-held harbour using collapsible canoes, slipping past patrols to plant limpet mines on ships. He is given command of a small group of volunteers, including Marine Clarke [Anthony Newley], whose nerves and resolve are tested as the mission looms.
He must navigate a clash of cultures within the unit: the eager, improvisational mindset Stringer embodies contrasts with the steady, by-the-book approach of Captain Hugh Thompson [Trevor Howard], a veteran officer who represents the old guard. Stringer, a temporary wartime officer, is full of enthusiasm for daring feats but lacks real experience in leading men or orchestrating complex operations, while Thompson has long felt passed over for promotion after a notable setback in the First World War.
Sergeant Craig [Victor Maddern] trains the men to follow Stringer’s directions, though Thompson disapproves of his command style and the flexibility Stringer insists upon. After a test mission ends disastrously, Stringer acknowledges his error and turns to Thompson, who then begins to whip the marines into shape with a more disciplined, practical approach.
Meanwhile, Marine Ruddock [David Lodge] confronts personal turmoil and goes AWOL. Thompson tracks him to his wife, [Beatrice Campbell], and finds her with a civilian lover, but pulls back when they insult him. He heads to the local pub for a drink and locates the missing Marine, and then—accepting the gravity of the situation—gives Ruddock enough time to confront the affair before driving him back to camp.
The raid proper is launched soon afterward by submarine HMS Tuna under the command of Lieutenant-Commander Alan Grieves [Christopher Lee]. As the canoeists near the Gironde estuary, a depth‑charge attack from a passing German patrol boat cripples Ruddock’s partner, forcing Thompson to volunteer to take his place. The raiders are dropped into the sea near the mouth of the estuary, Tuna submerges, and the long, arduous paddle begins. They push through a treacherous December night, moving by stealth and resting only when conditions allow, as the group makes their way roughly 70 miles upriver in Cockle Mk II collapsible canoes.
Four men reach the target and plant mines on several German cargo ships, enduring unforgiving weather and persistent hazards. The operation is technically successful, but the escape is grim: only Stringer and Clarke manage to break away, while Thompson, Ruddock, and the others are captured or killed on the way to the docks. When the prisoners refuse to reveal the mission’s details, they are executed by firing squad just as the mines explode. In the end, Stringer and Clarke walk away, accompanied by the apparitions of Thompson and the fallen, leaving a haunting image of sacrifice, resolve, and the cost of daring under wartime pressure.
