American David Ross (William Holden), a former tugboat captain now in the Canadian Army, is hastily commissioned in the Royal Navy and assigned to rotating command of ''W88'', a double-screwed salvage tug then in dry dock due to battle damage. His predecessor was a suicide. The slow, poorly armed tugs bring in "lame ducks," freighters crippled near Britain by German attacks. The main danger is from U-boat and aircraft.