Corvette K-225

In 1943, during the height of the Battle of the Atlantic, Lieutenant Commander MacClain's ship and most of his crew are lost to a ruthless U-boat attack. The submarine surfaces and machine-guns many survivors, leaving him grievously scarred and determined to avenge them. He resolves to continue the fight even from ashore and seeks a new command.

MacClain is offered duty ashore and is assigned a new Flower-class corvette. He befriends Joyce Cartwright, whose brother Dick was killed under his command, and their shared grief becomes a driving force behind his resolve.

The Donnacona, a Flower-class corvette, is christened and commissioned for escort duty. A crew of 65 joins the ship, including young officer Paul Cartwright, readying for dangerous tasks ahead.

Donnacona sets out as an escort to a convoy bound for England. The crew trains under threat of U-boat attack and prepares to sustain convoy defense in the rough Atlantic.

Early in the mission the Donnacona discovers a lifeboat filled with the dead, a grim reminder of the war at sea. The find sharpens the crew's resolve to protect the remaining merchants and to honor those lost.

A violent storm drives the Donnacona away from the main convoy, leaving the ship 300 miles from the Irish coast. The crew struggles to keep course and maintain contact with straggling vessels.

MacClain and the small group of ships head back to sea to stay with the surviving merchants. The Egyptian Star's captain relays that a submarine may be trailing them, heightening the sense of danger.

Luftwaffe bombers attack the screen, but a Fleet Air Arm Hawker Hurricane fighter launched from an escort ship drives them off. Meanwhile, the submarines below remain the chief threat as patrols tighten.

The Egyptian Star is torpedoed and sinks, escalating the danger to the escort force. MacClain answers with a disciplined depth-charge attack, sinking a U-boat in the process.

A second U-boat surfaces and damages the Donnacona badly. The crew fights back with deck gunfire, striving to keep the ship afloat while repairs are made.

MacClain attempts to ram the submarine as it dives; depth charges explode and sink it. He realizes it is the same U-boat that had machine-gunned his former crew, sealing a personal vendetta fulfilled.

With six surviving merchant ships, the Donnacona limps into Irish waters and heads for harbor. The crew is exhausted but buoyed by the prospect of safety and relief after their ordeal.

Before anchoring, MacClain is asked to sail the Donnacona past the other ships so that its crew may be saluted for bravery. The moment underscores the ship's hard-won reputation and the cost of their victory.