During the First World War, Lieutenant Richard Saville, Michael Rennie, a young Royal Navy officer on five days' leave, and Miss Lucinda Bentley, Wendy Hiller, a merchant's daughter from Portsmouth, strike up a conversation on the train up to London. Halfway through their journey, they miss their rail connection and decide to spend a romantic holiday in the countryside of southern England. When Saville proposes, she accepts, but on the day they are due to go back to Portsmouth, she changes her mind, asking Saville to realize that neither he nor she could bear being parted for the long periods he would be at sea. They part, seemingly forever.
Saville goes on to serve through the First World War and the inter-war years, and, by the early years of the Second World War, he is in command of a squadron of three cruisers on convoy duty in the Pacific. He receives a warning from a British merchantman just before it is sunk by the German raider Essen, but HMS Stratford, the squadron’s flagship, is too low on fuel for pursuit and the convoy cannot be left unguarded. Saville decides to stay with the convoy while his other two ships—HMS Amesbury and HMS Cambridge—chase after the raider. Cambridge then has to stop to pick up survivors from the merchantman, leaving the Amesbury on her own. Amesbury locates and attacks the Essen, scoring a major torpedo hit on the Essen’s bow, but is sunk with the loss of all but two hands, Petty Officer Wheatley and Signalman Andrew 'Canada' Brown. Brown is the son of a mother keen on the navy and thus knows more about naval tactics, strategy, and gunnery than most of his rank.
The Essen picks up the two survivors. Meanwhile, news of the Amesbury’s fate reaches Saville in the Stratford. Saville decides to take a risk and go after the Essen with Cambridge. While the Essen is anchored in a rocky lagoon for 36 hours to carry out repairs, Brown manages to escape to the heights around the lagoon with a rifle. Having won marksmanship prizes, he proceeds to pick off sailors working on the repairs, prompting the Essen’s captain to use his ship's anti-aircraft guns and then big guns in a vain attempt to dislodge Brown. Finally, he sends a party of marines to hunt Brown down, but just as they are about to kill him, they are recalled and the Essen departs. Brown collapses, seriously wounded.
As the Essen leaves the lagoon, she is caught and sunk by Saville's force. One of her survivors informs the British of Brown's exploits, which delayed repairs for 18 hours, thus enabling the British to catch up with them. A landing party is sent ashore from Saville's force to find him.
