It was the first British airborne operation using Airspeed Horsa gliders, and its target was the Vemork Norsk Hydro hydrogen electrolysis plant in Telemark, Norway which produced heavy water as a by-product.

The first pair suffered from navigational difficulties and severe weather, which resulted in the tow rope snapping and the first glider crash-landing, with its Halifax tug aircraft returning to base.

When Norway was invaded and occupied by Germany in April 1940 this obstruction ended; the Vemork plant was captured and began producing heavy water for the German atomic weapons programme.

[9] Additionally SOE decided to send an advance party of Norwegian agents into Telemark, and began intensively training a four-man team over the summer.

The party, code-named Operation Grouse, was led by Jens-Anton Poulsson and also included Knut Haugland, Claus Helberg and Arne Kjelstrup.

[13] The team landed at Fjarefit[14] on the Hardangervidda (a large wilderness avoided by the German forces)[15] and spent the next 15 days trekking towards Møsvatn, where they made contact with Skinnarland's brother, Torstein.

A suitable glider landing site was chosen 3 miles (4.8 km) south west of Møsvatn dam[17] and the team reconnoitred the area to help Combined Operations decide the best way to attack the plant.

[6] Because of the complicated and technical nature of the operation, which would see the plant rigged with explosives and then detonated, it was believed that a minimum of twelve to sixteen men was required, and that they would all have to be skilled engineers.

Cooper; it was provided with a special allotment of three Handley Page Halifax heavy bombers for the operation, which were the only British aircraft in existence at the time which were capable of towing Horsa gliders the distance required and then returning to their base.

[23] The SOE agents selected a landing zone for the sappers, which was approximately 5 kilometres (3.1 mi) from the Norsk Hydro plant and could not be observed by German patrols.

[3][23] Combined Operations initially suggested the men should fight their way to the Swedish border, but MI9 believed their chances of survival were better if they attempted to disguise themselves as Norwegians and travel in pairs.

[26] Training began in early October in Wales and was designed to be extremely arduous, not only to ensure that those who took part were at the peak of their physical and mental fitness, but also to eliminate any men unable to cope with the long marches and difficult terrain that would be encountered during the operation.

[22] On a second attempt to find the correct area, the combination flew into thick clouds approximately 40 miles (64 km) northwest of Rjukan, and ice began to form on both aircraft and glider, as well as the towing rope connecting the two.

[34] After contacting more of the local residents, the soldiers asked for their assistance in escaping to Sweden, but Haaheller and others convinced them that this would be impossible, as it would involve travelling across the entirety of Norway.

[35] The Norwegians also burned all of the maps and documents from the glider crash before the Germans could arrive, and managed to keep the entire operation secret for almost twenty-four hours.

[39][44] The two RAAF pilot and copilot of the glider, Norman Davies and Herbert Fraser, were killed instantly and the rest were injured to varying degrees, one of whom, 25 year old Driver Ernest Pendlebury, died soon after.

After searching for some time, they arrived in the hamlet of Helleland about 2 miles (3.2 km) from where they had crashed, and made contact with one of the local residents, Trond Hovland.

[46] A party from the Norwegian Labour Service arrived at the crash site shortly afterwards and helped tend to the injured soldiers[44] who burned all of their sensitive documents and materials.

Unwilling to abandon the wounded and doubting their chances of escaping to Sweden, Lieutenant Allen chose to surrender, believing that they would be treated as POWs.

With this, and the fact that sabotage equipment was found, Colonel Probst, the head of the Wehrmacht in the Stavanger district, had all survivors executed as partisans at the German camp at Slettebø near Egersund.

[44][47] The Allies were unaware of the fate of the operation until they intercepted a German communiqué stating that two gliders and one aircraft had been forced down, and the crews engaged and annihilated.

Of the soldiers from the first glider, three of the four injured men were tortured by the Gestapo and later killed by Stabsarzt [Dr] Werner Fritz Seeling who injected air into their bloodstreams.

[53] The Wehrmacht believed they should be treated in accordance with the Geneva Convention, but in the end the survivors were interrogated and executed within a few hours of their capture at the German barracks at Bekkebø.

[54] German reprisals started instantly and 200 armed Gestapo agents swept into Rjukan where they arrested 21 Norwegians for questioning,[55] but the members of Operation Grouse slipped away into the wilderness of Hardangervidda.

[56] They later contributed to the successful Operation Gunnerside in February 1943, when a small team of Norwegian SOE agents were parachuted into the area and destroyed much of the heavy water equipment in Vemork plant.

[57] The plant eventually resumed operation, but further bombing raids ensured it produced little heavy water for the German atomic weapons programme.

[59] When 1st Airborne Division arrived in Norway in May 1945, they were informed of the fate of the prisoners, and cooperated with the Norwegian government to have the men buried with full military honours.

[22][31][verification needed] The commander of the German forces in Norway – Generaloberst Nikolaus von Falkenhorst – was also found guilty of two of the Freshman deaths during his war crimes trial.

[41] Sven Hedin intervened on his behalf on 4 December 1946, with the argument that von Falkenhorst had likewise striven to pardon the ten Norwegians condemned to death.