Kapitein Ericson is de bevelhebber van een Brits orlogsschip dat tijdens de Tweede ereldoorlog andere schepen moet beschermen op de Atlantische Oceaan. Op zijn vorige schip heeft Ericson veel manschappen verloren. Nu moet hij opnieuw een keuze maken tussen het welslagen van zijn missie en het leven van zijn manschappen.
Opening in the autumn of 1939 just as the Battle of the Atlantic begins, Lieutenant Commander George Ericson, after service in the British Merchant Navy, is recalled to the Royal Navy as a member of the RNR and given command of HMS Compass Rose, a newly-built Flower class corvette intended for convoy escort duties. His officers Lockhart and Ferreby, are both newly-commissioned and without experience at sea. The new First Lieutenant, James Bennett (Stanley Baker), is an abusive martinet.
Despite these initial disadvantages, the ship’s company gains hard experience and becomes an effective fighting unit. At first their worst enemy is the weather, since German submarines lack the range to attack shipping far into the Atlantic. When French ports fall to the Germans, U-boats can attack convoys anywhere in the Atlantic – ironically making bad weather the convoys’ greatest advantage. The first lieutenant is put ashore, the junior officers mature and the ship crosses the Atlantic many times escorting convoys, often in brutal weather. They witness the sinking of many merchant vessels they are charged with protecting and the tragic deaths of merchant navy crewmen. A key scene involves Ericson’s decision to carry out a depth charge attack even though the blast will kill merchant seamen floating in the water. After close to three years of service, including one U-boat sunk, the Compass Rose is herself torpedoed and her crew forced to abandon ship. Most of the crew are lost to drowning and hypothermia. Taking to a couple of liferafts, Ericson survives this ordeal along with his First Lieutenant, Lockhart (Donald Sinden), and with the few crew left are picked up the next day.
Ericson is promoted to full commander, and together with Lockhart his now-promoted “Number One”, Ericson takes command of a new ship, HMS Saltash Castle a new frigate (though in the film the frigate was portrayed by a Castle-class corvette), and with Ericson leading an anti-submarine escort group they continue the monotonous but vital duty of convoy escort. Late in the war, while serving with the Arctic convoys, they doggedly pursue and sink another U-boat, marked as U-53 (in reality, sunk by HMS Gurkha in February 1940 with the loss of all hands), Saltash Castle’s only ‘kill’. As the war ends the ship is shown returning to port, as guard to a number of German submarines that have surrendered.