Cammell Laird launched Pathfinder from their Birkenhead yard on 16 July 1904, the lead ship of a class designed to shepherd destroyer flotillas through the North Sea. She was 379 feet long, displaced just under 3,000 tons, and could make 25 knots when her twelve Normand boilers were pushed hard. The Admiralty had specified light armor and modest guns, because nobody expected a scout cruiser to fight anything bigger than another destroyer. She had originally been ordered as Fastnet, but somebody in the Admiralty changed their mind before the keel was laid. By August 1914 she was leading the 8th Destroyer Flotilla out of Rosyth, with Captain Francis Martin-Leake in command. The Firth of Forth was the busiest naval anchorage in Britain, and the new German submarines were exactly the kind of threat scout cruisers were meant to look out for. Nobody quite understood yet how serious that threat was.

Kapitänleutnant Otto Hersing took U-21 deep into the Firth of Forth in the first week of September, slipping past the Carlingnose Battery beneath the Forth Rail Bridge. The shore gunners spotted his periscope and opened fire, but Hersing dove and slid away. Overnight he withdrew south, patrolling the coast from the Isle of May. On the morning of 5 September he watched through his periscope as Pathfinder steamed south-southeast with destroyers in trail. At midday the destroyers peeled off back toward the Isle of May. Pathfinder continued alone. When she came back on her return patrol, Hersing was waiting. At 1543 he fired a single 50 cm Type G/6 torpedo from 2,000 yards. The men on Pathfinder spotted the wake at 1545. Lieutenant-Commander Favell ordered the starboard engine astern and the wheel hard a port, trying to comb the torpedo's track. He was two minutes too late.

The torpedo struck under the bridge and the forward magazine exploded almost instantly. Everything ahead of the bridge was destroyed. Pathfinder broke in two and went down so quickly that her crew had no time to launch lifeboats. The remains of a davit and a length of rope still lie tangled on the wreck today, frozen mid-attempt. Of roughly 270 men aboard, only about 20 survived. Four more died of their injuries and exposure in the days after, and were buried at Dalmeny in Fife and at Warriston near Edinburgh. One unidentified Pathfinder sailor lies at Dunbar, overlooking the water where his ship went down. The British writer Aldous Huxley was staying at Northfield House in St. Abbs that afternoon, and saw the column of smoke rise from the sea. He was twenty years old.

The Admiralty insisted publicly that Pathfinder had struck a mine. Submarines, they maintained, simply could not sink a major warship with a single torpedo. The position was politically convenient and technologically wrong. The Scotsman published an eye-witness account from an Eyemouth fisherman who had helped pull survivors from the water and knew exactly what he had seen. The truth came out quickly. By the end of the war, U-boats would sink millions of tons of British shipping, and Otto Hersing would go on to torpedo HMS Triumph and HMS Majestic at Gallipoli. But Pathfinder was the moment everything changed. Two minutes between sighting the wake and the explosion. A ship and her crew gone. A new way of war announced to anyone paying attention.

Pathfinder lies in about 65 metres of water off St. Abbs Head, broken roughly in two, with the bow section largely destroyed by the magazine blast. Recreational divers visit her in summer, and the Royal Navy honoured the centenary of her loss on 5 September 2014. The coast above the wreck is one of the most dramatic stretches of the Berwickshire seaboard, with St. Abbs Head's sea cliffs and seabird colonies rising sharply from the water. A nameless sailor watches from Dunbar. Two hundred and sixty more lie below.