[2] Seal was laid down at the Chatham Dockyard on 9 December 1936, launched on 27 September 1938 and commissioned into the Royal Navy on 28 January 1939.

On the day of her first successful deep dive, 1 June 1939, news arrived of the loss of HMS Thetis undergoing trials at Liverpool, a personal setback for the crew who had lost many friends.

[3] On 4 August, she sailed to China to join HMS Grampus and Rorqual via Gibraltar, Malta and the Suez Canal.

Back in the North Sea, she carried out one patrol near the Dogger Bank and received her first attack from German aircraft.

One night in February, Seal was given an extra set of personnel – an armed boarding party – and was assigned to take part in the hunt for the German tanker Altmark.

Admiral Horton met Seal on one of her returns to Rosyth and commented, "You're too damn clean for a war-time boat.

There were four merchant ships in the harbour, but they all carried neutral flags; Lonsdale's requests to attack a seaplane base and land a shore party to sabotage the railway met with firm refusals; and the German naval craft they encountered had too shallow a draught for Seal's torpedoes to hit.

The disappointed crew returned to Rosyth, narrowly escaping a torpedo attack at the same place and time as that in which HMS Thistle was lost.

[5] Having been at sea for a year, and suffering some damage from a scrape with a merchant ship, Seal was due to return to dry-dock at Chatham.

On entering the Skagerrak, she met HMS Narwhal just leaving the area after having stirred up German defences by scoring six hits with six torpedoes.

Seal was running at shallow depth to maintain speed and conserve fuel, when she was spotted by a German Heinkel He 115 on 4 May at about 02:30.

There were too many hours of daylight left, and the Kattegat was too shallow to allow a submarine as large as Seal to go deep and run for it.

The crew's evening meal was catapulted round the mess rooms and the boat tilted bow upwards at about 10 degrees.

All the watertight doors were quickly sealed and all crew accounted for, after two who had been trapped in the after end of the boat managed to make their way to the control room.

[9] At 10:30 pm, the ballast tanks were blown empty and the main motors started, but the stern stayed firmly stuck on the sea bed.

The engineers found they could open a salvage-blow[clarification needed] and a final attempt was made to raise the submarine.

After the pressure was released, the fresh air caused blinding headaches to the crew, who had suffered oxygen deprivation.

With Seal under bombing and gunfire attack from the air,[13] unable to dive and without motive power, some men wounded and no remaining defences, Lonsdale had no alternative but to surrender.

Admiral Rolf Carls believed Seal was a war-winning asset and insisted that she be made operational, despite the probability that three superior new German U-boats could be built for the same cost.

Practice runs revealed so many snags and the financial costs were so unrealistic, that by the middle of 1943 she was paid off, stripped, and abandoned in a corner of Kiel dockyard.

The significant value derived was the realisation that the British contact pistol torpedo detonator was of superior design and its introduction into the German navy.

Petty Officer Barnes took part in a mass break out and with Sergeant Major George Briggs of the 15/19 Hussars managed to make contact with the Polish underground.

[19] One of the engineers, Don "Tubby" Lister, made a series of escapes and was eventually sent to Oflag IV-C at Colditz Castle.

Realising how hard it would be to escape from there, he and another ERA (Engine Room Artificer), W. E. "Wally" Hammond (from the sunken submarine HMS Shark), insisted on being moved on the grounds that they were not officers.