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Lieut. Commander MacClain and the remainder of his crew return to Canada after his ship, a corvette, is destroyed by a German submarine's torpedo while accompanying a convoy. Although he is being offered a leave by his admiral, Mac insists on returning to sea as soon as possible in order to avenge his men, as many were machine-gunned to death by the German U-boat. While waiting for his new ship, Mac meets Joyce Cartwright, whose brother Dick, an officer, was killed under Mac's command. Though she initially blames Mac for her inexperienced brother's death, Joyce and Mac begin seeing each other and quickly fall in love. Mac's new ship is christened the HMCS Donnacona , and it is soon commissioned for service with a crew of sixty-five, including officer Paul Cartwright, Joyce's younger brother, who complains to his sister about the inability of the new crew to satisfy their commander. The next convoy, under the direction of Commodore Ramsay, leaves Canada for Europe with a cargo which includes gasoline, tanks and airplanes. German submarines are sighted off the coast of Newfoundland, near a point known as "Ethel," and after a non-convoy ship is torpedoed in that area, Mac's ship comes across a lifeboat filled with dead sailors from the sunken vessel. On its ninth day at sea, the convoy is hit by a ocean storm, and the Donnacona becomes separated from the other ships. Three hundred miles from the Irish coast, Mac meets up with some of the convoy's other lost ships, including the tanker Egyptian Star . The captain of the tanker, however, warns Mac that he thinks his ship is being followed by German submarines. Their ships soon come under attack from a squadron of German bombers, but the Germans are repelled by one of the convoy's fighter planes. That night, the Egyptian Star is sunk by a torpedo, and the Donnacona engages the responsible submarines. The first submarine is sunk by depth charges, but the Donnacona is crippled in a surface battle with a second U-boat. The injured Mac attempts to ram the submarine, but when it begins to dive, Paul and seaman Stooky O'Meara set off depth charges, which strike the sub. As it breaks up, Mac recognizes the German U-boat as the one which had machine-gunned to death his men in the prior convoy. The corvette, along with six merchant ships, arrives safely in Ireland, but before it sets anchor, it is asked to sail past the other ships in the harbor, so that its crew may be saluted for their bravely.