During the second world war, Nobel price winner Werner Heisenberg is the head of the nazis atomic project. Heisenberg needs heavy water for the reactor he is building, only produced by Norwegian Hydro at Vemork. The creator of the heavy water factory, scientist and resistance fighter Leif Tronstad, come to be sucpicious of the german purchase of heavy water and contacts English intelligence.

Leif Tronstad assembles a sabotage group named Grouse. The aim is to assist the English commandos to blow up the water plant at Vemork. In Germany, Werner Heisenberg continues his work on the atomic reactor. Grouse is dropped over Hardangervidda and prepare a landing for the Englishmen.

The English mission fails and the allies now wants to bomb the heavy water plant. Leif Tronstad persuades them to submit a Norwegian special force group to blow up the factory. Tronstad collect saboteurs, name the group Gunnerside and start training.

Gunnerside lands on Hardangervidda and joins grouse. The twentyseventh of February, 1943, the 9 saboteurs skies down the mountain in the direction of Vemork to blow up the heavy water plant.

The Germans are hard at work rebuilding the heavy water factory after the sabotage. CEO Henriksen at Norwegian Hydro discovers that he is dangerously close to be labeled as a war collaborator and tries negotiate his way out. In England the Americans decide to bomb Rjukan and the factory at Vemork.