The action commences in 1939. Lieutenant-Commander George Ericson, a Merchant Navy and Royal Naval Reserve officer, is recalled to the Royal Navy and given command of the fictitious Flower-class corvette HMS Compass Rose , newly built to escort convoys. His officers are mostly new to the Navy, especially the two new sub-lieutenants, Lockhart and Ferraby. Only Ericson and the petty officers are in any way experienced.
Despite these initial disadvantages, the ship and crew work up a routine and gain experience. Bennett, the first lieutenant, a mean and shirking disciplinarian with a penchant for bullying and canned sausages (snorkers), leaves the ship ostensibly for health reasons, and the junior officers are able to mature, with Lockhart gaining promotion to first lieutenant.
The crew cross the Atlantic many times on escort duty in all kinds of weather, often encountering fierce storms in one of the smallest ships to provide escort services to the Allied convoys. The men endure the ship's constant rolling and pitching in the huge waves, freezing cold, the strain of maintaining station on the convoy on pitch-black nights and the fear that at any second a torpedo from a German U-boat could blow them to oblivion. Somehow the tradition of the Royal Navy and the knowledge of the importance of their work carries them through.
They continue the monotonous and dangerous but vital duty of convoy escort and after one particularly difficult convoy they use all their hard won knowledge to sink a German submarine. They are nearly sunk several times until in 1943 they are finally torpedoed and forced to abandon ship. Most of the crew die in the freezing waters, but Ericson, Lockhart, Ferraby, and a few others are rescued the next day, but Ferraby suffers a break down forcing him to go to hospital.
Ericson, now promoted to commander, and Lockhart, now a lieutenant-commander, take command of a new ship, the fictitious River class frigate HMS Saltash . (In the film adaptation, the ship is called Saltash Castle and is portrayed by the Castle class corvetteHMS Portchester Castle
, as no River class vessels were available.)
The Royal Navy is now finally gaining the upper hand over the U-boats and Saltash adds to the growing number of kills due to Ericson's determination and patience.
In chapter seven the ship receives a message ordering it to "remain on patrol in vicinity of Rockall" as the end of the Second World War approaches in 1945, a well-known quote.
When the war ends, the ship returns to port as a guard to several German submarines that have surrendered.
A secondary plotline concerns Lockhart's poignant romance with a beautiful Women's Royal Naval Service officer.