Norville, on the coast of occupied France. At a time when German U-Boats are inflicting heavy damage on allied shipping, a French resistance officer learns the port is being used as a submarine base, but is very lightly guarded. He passes the information to British Intelligence, who begin planning a surprise attack, using the 95th Brigade. Aware that any leak to the enemy would mean disaster, they appoint Major Richards to ensure secrecy.
Richards joins the Brigade at Newby, and discusses security with the Brigadier. The Brigade is being moved to Watercombe, near Westport, and news has already leaked out. Richards suggests censorship of troops' communications. In the meantime, the men are to be issued with tropical equipment to make them believe they are heading to the Near East.
Later, the men enjoy a performance by dancer Miss Clare, and young Private Tommy visits her backstage, where she is attended by 'Ma' Webster. When Ma leaves the room, he reveals that he will be stationed at Watercombe, and she promises to try and rearrange her schedule to meet him at Westport. Ma returns and Tommy leaves. Under pressure from Ma, who threatens to cut off her supply of cocaine, Miss Clare passes on what Tommy told her.
Next day, the Brigadier lectures the troops on maintaining secrecy. Meanwhile, a German general learns of the 95th Brigade's move to Watercombe. Two agents, Numbers 23 and 16, are dispatched to England.
The troops take a train to Watercombe. In the bar, No. 16 listens as the men discuss their postings, but one soldier becomes suspicious and informs the Brigadier. The spy's cover is blown when parachute harness marks are seen on his shoulders.
No. 23 successfully meets his contact, a bookseller named Mr. Barratt, and is given the identity Davis and a story about being bombed out in Cardiff, then sent to Watercombe. Meanwhile, Barratt's assistant, Beppie Leemans, a Dutch refugee, is visited by Private John, who is about to join the Brigade.
At Watercombe, the men undergo rigorous training. Mr Davis is spotted watching the troops, but when challenged, he leaves. Officers are surprised to see an enemy reconnaissance plane.
John ignores censorship rules and posts a letter to Beppie, inviting her to Watercombe that Sunday, which Barratt reads.
Following up suspicious reports about Davis, Richards visits his hotel and spots Tommy with Miss Clare. He calls Westport police, who visit 'Ma' and find transmitting equipment in Miss Clare's dressing room. At the hotel, Miss Clare is arrested. Later, Richards is informed Davis has left.
Davis is reassigned as a labourer at an ordnance depot. He helps an ATS officer fix a flat tire, and she invites him to a dance. There he hears about a priority order of equipment for the 95th Brigade.
Beppie is blackmailed by Barratt, who threatens harm to her parents in Rotterdam, into obtaining information about the Brigade's plans from her boyfriend. She reveals they are waiting for 'mosaics' - air photographs - but realising the significance of her informing, she kills Barratt. Davis witnesses the murder, and knocks her out, then turns on the gas to make her death appear a suicide.
In London, another spy intercepts the mosaics by switching briefcases with a careless officer. The photographs are copied and the case returned.
Plans are finalised for the raid on Norville. Meanwhile, the photographs reach enemy hands; the Germans realise Norville is the target and prepare defences. The Brigade reaches the coast and launches three landing parties. All meet strong resistance from the Germans, and suffer heavy casualties. The mission is ultimately successful, with munitions dumps and one enemy submarine destroyed, but at a very great cost.
Later, two upper-class gentlemen on a train discuss a forthcoming military operation, as Davis, in the same carriage, listens in.
