Undine was built by Vickers Armstrong at Barrow-in-Furness on the Cumbrian coast, the yard that would supply most of Britain's submarine force through the twentieth century. Her keel was laid on 19 February 1937. She was commissioned on 21 August 1938 with the pennant number N48 - the first of a new class of small, agile boats designed initially as training submarines. They were a little under 200 feet long, slow on the surface, slower below, but small and quiet enough to slip into shallow coastal waters where bigger boats could not safely go. When war came in September 1939, the U-class was suddenly an offensive weapon. The training boats went on patrol.

At the outbreak of the Second World War, Undine was part of the 6th Submarine Flotilla. Between 26 and 29 August 1939, with war hours away, the flotilla deployed from peacetime ports to its war bases at Dundee in Scotland and Blyth in Northumberland. Their job was to watch the German coast from the inside of the North Sea. Heligoland, the small red rock of an island off the mouths of the Elbe and Weser, was the obvious focus - it was the forward base of the entire German naval defense. Patrolling near Heligoland in early 1940 was as close to enemy waters as the Royal Navy could get without actually entering the Jade or the Elbe themselves. The minefields were thick. The German patrols were constant.

Undine was on her fourth war patrol when her sonar gave up. A small leak in the hydrophone fitting was enough to silence the boat's ears entirely. The crew kept going. At 0940 on 7 January, German auxiliary minesweepers M-1201, M-1204, and M-1207 spotted her. Undine launched torpedoes at the leading vessel and missed. The Germans counterattacked, dropping depth charges and forcing the submarine to dive. With no sonar, the captain had no way to track his attackers underwater except by the muffled thumps of the charges. He took the boat to fifty feet and waited. After five minutes of silence, he chose the only option left: raise the periscope and look. The Germans had been waiting for exactly that. A heavy explosion - probably from a final depth charge dropped close aboard - tore into the hull and made the boat untenable.

The crew got out. They opened the main vents to flood the boat and went into the cold January North Sea, where the Germans pulled them aboard the minesweepers. Of Undine's company of 27 men, every single one survived to spend the rest of the war as prisoners. As lost-submarine stories go, that is rare and unequivocally good news. Their boat, the lead ship of the class that would eventually number 49 submarines and play a major role in the Mediterranean war, sat on the bottom of the southern North Sea, perforated and silent. The next U-class boat to be lost would not be so lucky. Undine's story was the opening chapter of a hard underwater war that the Royal Navy would, in the end, win.