In 1943, 'Mac' MacClain, Canadian Navy, has lost his ship and many men to a German torpedo. While waiting for a new ship, he befriends Joyce Cartwright, sister of one of his dead officers. We follow the building and launch of new Corvette K-255, the 'HMCS Donnacona'. And who should be Mac's new subaltern but Joyce's other brother Paul, fresh out of the academy. Mac will do his best to make a good officer of Paul...if they both survive their hazardous sea duty.