On 5 January 1940 HMS Starfish (Lt. Thomas Anthony Turner, RN) sailed for patrol in the Heligoland Bight from Blyth. At 1040 hours on 9 January 1940 she attacked the German minesweeper M 7 (Korvettenkapitän Neitzel) off the German North Sea coast in the Heligoland Bight in position 55°00'N, 07°10'E. However this attack failed because, due to drill error, the torpedoes were not fired and remained in the tubes(!).
She returned for second attack but the hydroplanes jammed and the CO decided to bottom for the remainder of day to carry out repairs in about 27 metres of water. According to Turner in his postwar report, the German minesweeper meanwhile had located her and dropped 2 depth charges which did no damage, but at 1050 hours one of the crew asked for permission to restart one of the Sperry motors to prevent the gyro from wandering, it was granted and, no sooner the motor had been started, 4 depth charges were dropped right on top of the boat, fairly close aboard, causing widespread damage.
At 1440 another depth charge attack was carried out, 20 of these falling fairly close to the hull, shearing rivets and hull plates which began leaking. by 1800 hours the situation inside was serious, the engine room crankcases and starboard main motor bearings were flooded, the torpedo trenches and bilges were full, water was pouring through the starboard engine clutch and lapping the starboard main motor casing.
Starfish laid on the bottom until the Commanding Officer, having formed the opinion the enemy was not likely to leave the vicinity in the near future, at 1820 hours gave the order to surface. In order to accomplish this, the sub was forced to drop the ballast keel, barely making it due to loss of HP air and water in the hull, coming up at a 45° angle. She sank very shortly with no loss among the crew, which was picked up by the waiting ships and taken as POW's.

According to the German minesweeper’s report, M 7 had detected the submarine in Grid AN 9385 at 1040 and carried out depth charge attacks at 1053, 1140, 1256, 1510, 1518 and 1518 hours. At 1932, the submarine was forced to the surface and came under fire from its MG C/30 guns. About ten men jumped in the water followed shortly after by the remainder of the crew. In the meanwhile the minesweeper M 5 had also arrived on the scene. M 7 came alongside and attempted to board the submarine but it sank from the stern. In July 1940, the diving vessel Oldenburg assisted by submarine chasers from the 17th Flotilla attempted to locate the wreck to recover secret documents but with little success. Circumstances of her loss were learnt in Great Britain when a Dutch submarine officer, Lt. Steinmetz, who had befriended one of her survivors (Lt. G. Wardle) escaped from a German PoW camp.