Shortly after the outbreak of hostilities, the Royal Navy Corvette Compass Rose is fitted out and commissioned for convoy escort duty. Captain Ericson - the only professional seaman among the officers - greets new arrivals Lockhart and Ferraby, who are quickly put to the test by the brutish 1st Lieutenant Bennett.

Compass Rose leaves harbour for sea trials and three weeks of training for her inexperienced crew. The radar operation training amuses the crew, but a sharp lesson in reality follows when they rehearse depth-charge runs.

Lieutenant Morell joins the crew prior to their receiving active service orders to escort a convoy across the North Atlantic. This proves a baptism of fire, as they are battered by storms and weather the incessant radar search for U-Boats and the unending waterlogging of crew and quarters alike.

As they sail back into their home port of Liverpool, the radio brings news of the slow retreat of the British Expeditionary Force towards Dunkirk. Bennett returns from shore leave drunk and insults the other junior officers. They persuade him to transfer off the ship with a feigned illness, and Lockhart is promoted in his place.

After the port of Brest falls to the Nazis, they encounter their first U-Boat attack. The crew combat efficiently, but two of the convoy's merchant ships are lost. Survivors are taken aboard Compass Rose, where Lockhart tends to them in the absence of a Medical Officer.

The year passes with the enemy growing stronger. Compass Rose returns to port for a re-fit. Chief Petty Officer Tallow takes Chief Engineer Watts home with him and introduces him to his widowed sister, who shares a beer with them both.

Back on Compass Rose, they escort convoys on the Gibraltar run, suffering successive sinkings as they are shadowed by increasing numbers of U-Boats, and filling the decks with survivors. By the time two Royal Navy Destroyers arrive to help, the Corvette is 'full' and the convoy is being pursued by an eleven-strong U-Boat pack - one for every ship left in the convoy. Another sinking sees a U-Boat stop underneath surviving merchant seamen floating in the water, and Ericson decides to steam through them in order to depth-charge the submarine. His anguish at sacrificing British sailors is made all the worse by the lack of success. In Gibraltar, three surviving merchant Captains offer him their thanks and support.

Back at sea, the Compass Rose is forced to stop for noisy repairs, entailling a tense overnight wait. The relief at being underway is soon forgotten as they encounter, but eventually sink, a surfaced U-Boat.

Returning to their home port, they find it bombed and afire. Tallow takes Ward ashore to find his home bombed and his sister killed. Meanwhile, Morell discovers his wife has been unfaithful, and Lockhart breaks off his budding relationship with Wren Julie Hallam, blaming the war.

At night, as Compass Rose heads out, it is attacked and sunk by a U-Boat. Morell, Tallow, and Watts are lost with many of the crew, and Ferraby has a breakdown. Ericson and Lockhart act to keep others alive long enough to be rescued.

Ericson is promoted to Lieutenant Commander and given command of the Frigate Saltash Castle. In a London restaurant, Ericson asks Lockhart to join him as first officer, and Lockhart loyally accepts rather than take a ship of his own. They join the Saltash Castle as it finishes its fitting out.

Escorting a convoy in the Baltic, Saltash Castle encounters a U-Boat, but loses it again after a depth-charge run. Lockhart assumes it is either sunk or escaped, but Ericson is determined not to let it get away. Weakened by the exhaustive search, Ericson approaches the ship's doctor who gives him pills to keep him awake. Suddenly, the radar makes contact with the U-Boat and he returns to the bridge to lead the depth-charge run. Damaged, the U-Boat surfaces and Ericson orders his crew to open fire. After a short battle the U-Boat sinks and its crew abandon ship. The Saltash Castle picks up the surviving German submariners.

After destroying only two submarines, Ericson and Lockhart end their war by sailing past the surrendered Nazi U-Boat fleet.