His Majesty's Ship Royal Oak (pennant number 08) was a Revenge-class battleship of the British Royal Navy, torpedoed at anchor by the German submarine U-47 on 14 October 1939. Launched in 1914 and completed in 1916, Royal Oak first saw action at the Battle of Jutland. In peacetime, she served in the Atlantic, Home and Mediterranean fleets, coming under accidental attack on more than one occasion. The ship became the centre of worldwide attention in 1928 when her senior officers were controversially court-martialled. During a twenty-five year career, attempts to modernise Royal Oak could not address her fundamental lack of speed, and by the start of the Second World War, she was no longer suited to front-line duty. In 1938, Royal Oak was made flagship of the Second Battleship Squadron based in Portsmouth. On 24 November 1938, she returned the body of the British-born Queen Maud of Norway to a state funeral in Oslo, accompanied by her husband King Haakon VII. Paying off in December 1938, Royal Oak recommissioned the following June, and in the late summer of 1939 embarked on a short training cruise in the English Channel in preparation for another 30-month tour of the Mediterranean. As hostilities loomed, she was instead dispatched north to Scapa Flow, and was at anchor there when war was declared on 3 September.

In October, Royal Oak joined the search for the German battleship Gneisenau, which had been ordered into the North Sea as a diversion for the commerce-raiding pocket battleships Deutschland and Graf Spee. The search was ultimately fruitless, particularly for Royal Oak, whose top speed, by then less than 20 knots (37 km/h), was inadequate to keep up with the rest of the fleet. On 12 October, Royal Oak returned to the defences of Scapa Flow in poor shape, battered by the North Atlantic storms: many of her Carley floats had been smashed and several of the smaller calibre guns rendered inoperable. The mission had underlined the obsolescence of the twenty-five year old warship. Concerned that a recent overflight by German reconnaissance aircraft heralded an imminent air attack upon Scapa Flow, Admiral of the Home Fleet Charles Forbes ordered most of the fleet to disperse to safer ports. The Royal Oak however remained behind, her anti-aircraft guns still deemed a useful addition to Scapa's otherwise scarce air defences.

Scapa Flow made a near-ideal anchorage. Situated at the centre of the Orkney Islands off the north coast of Scotland, the natural harbour, large enough to contain the entire Grand Fleet, was surrounded by a ring of islands separated by shallow channels subject to fast-racing tides. The threat from U-boats had long been realised, and a series of countermeasures were installed during the early years of the First World War. Blockships were sunk at critical points, and floating booms deployed to block the three widest channels. Operated by tugboats to allow the passage of friendly shipping, it was considered possible—but highly unlikely—that a daring U-boat commander could attempt to race through undetected before the boom was closed. Two submarines that had attempted infiltration during the war had met unfortunate fates: on 23 November 1914 U-18 was rammed twice before running aground with the capture of her crew, and UB-116 was detected by hydrophone and destroyed with the loss of all hands on 28 October 1918.

Kriegsmarine Commander of Submarines Karl Dönitz devised a plan to attack Scapa Flow by submarine within days of the outbreak of war. Its goal would be twofold: firstly, that displacing the Home Fleet from Scapa Flow would slacken the British North Sea blockade and grant Germany greater freedom to attack the Atlantic convoys; secondly, the blow would be a symbolic act of vengeance, striking at the same location where the German High Seas Fleet had surrendered and scuttled itself following Germany's defeat in the First World War. Dönitz hand-picked Kapitänleutnant Günther Prien for the task, scheduling the raid for the night of 13/14 October 1939, when the tides would be high and the night moonless.

At 00:58 U-47 fired a salvo of three torpedoes from its bow tubes, a fourth lodging in its tube. Two failed to find a target, but a single torpedo struck the bow of Royal Oak at 01:04, shaking the ship and waking the crew. Little visible damage was received, though the starboard anchor chain was severed, clattering noisily down through its slips. Initially, it was suspected that there had been an explosion in the ship's forward inflammable store, used to store materials such as kerosene. Mindful of the unexplained explosion that had destroyed HMS Vanguard in Scapa Flow in 1917, an announcement was made over the Royal Oak's tannoy system to check the magazine temperatures, but many sailors returned to their bunks, unaware that the ship was under attack.

Prien turned his submarine and attempted another shot via his stern tube, but this too missed. Reloading his bow tubes, he doubled back and fired a salvo of three torpedoes, all at Royal Oak, and this time he was successful: at 01:16 all three struck the battleship in quick succession at her amidships.
A series of explosions ran through the ship, followed by an inrush of seawater. The ship immediately listed some 15°, sufficient to push the open starboard-side portholes below the waterline. She soon rolled further onto her side to 45°, hanging there for several minutes before disappearing beneath the surface at 01:29, 13 minutes after Prien's second strike. 833 men died with the ship, including Rear-Admiral Henry Blagrove, commander of the Second Battleship Division. The admiral's wooden gig, moored alongside, was dragged down with Royal Oak.