In November 1940, the tanker MV San Demetrio is part of a convoy crossing the Atlantic in the teeth of the U-Boat wolf-packs. Her mission is to collect badly needed petrol from Galveston, Texas and return it to beleaguered Britain. Her captain, George Waite, is a no-nonsense old-school merchant mariner with a hardy and experienced crew from all over the UK. Prominent among them are Greaser John Boyle and his brother-in-law, Messboy John Jamieson. The officers, led by Chief Engineer Pollard and Second Officer Hawkins are also determined and resourceful.After an uneventful trip out to the Gulf of Mexico, Waite recruits an American seaman, 'Yank' Preston, to join the ship for the return voyage. While the ship is taking on fuel, the undisciplined Preston has several run-ins with Boatswain Fletcher, but is accepted by the rest of the crew. On the return voyage, a German raider is spotted in mid-Atlantic. Immediately, and knowing that, outgunned, an attack is suicidal but will give the convoy a chance, the escort vessel HMS Jervis Bay advances at full speed to engage the enemy. In the one-sided exchange of gunfire, Jervis Bay is sunk with all hands and the San Demetrio is hit and catches fire. .With thousands of tons of petrol about to go up in flames, the Captain orders his crew to abandon ship and they leave in the lifeboats. Some are picked up by other ships in the convoy, but one lifeboat is missed. The crew sail and row the lifeboat through the night and the next day, in stormy seas and freezing conditions and with limited food and water supplies. Boyle has been gravely injured in the escape from the San Demetrio, but courageously insists on doing his share, despite Jamieson's concerns.With hope giving way to despair, the crew sight a ship in the distance, and are amazed to see, as they get closer, that it's the San Demetrio. The ship is still on fire, and could blow up any second, but the situation in the lifeboat is dire and after a short debate, the crew decide to re-board her. What follows consists of their heroic struggles, led by Pollard and Hawkins, to put the fires out, rewire the electrics, provide hot food for the freezing men, shift the remaining oil to improve the trim of the ship and tend to Boyle, who soon dies from his injuries. Preston's positive attitude and ingenuity are a key factor and he earns the respect of the boatswain and the officers. Pollard even manages to get the engines restarted and rig up a manual steering system to replace the one destroyed by shellfire.Despite not knowing their exact position, the crew decide to try to complete the journey back to the UK, on the grounds that they came all this way to fetch the fuel, and there doesn't seem much point in taking it back to the States. They battle their way through more storms until they reach the coast of Ireland. They are offered some help by a tug, which they scornfully refuse - they've managed several thousand miles already without help. This means, they discover, that they are entitled to the full salvage money for bringing the ship in. In the final scene, the crew's enterprise is praised by officialdom, and the San Demetrio's ensign is awarded, as a mark of respect, to John Boyle's widow.