A German scientist seeks heavy water from a Norwegian factory to create a nuclear chain reaction and a professor escapes occupied Norway to warn the Allies about a Nazi attempt to build an atomic bomb.

Leif Tronstad is working with the War Ministry in London and helps draw up a plan to destroy the factory at Rjukan. Werner Heisenberg travels to Copenhagen to meet his Danish mentor, Prof Niels Bohr.

The two gliders from the 'Freshman' expedition have crashed into the Norwegian mountains and the 'Grouse' team members remain in hiding. In Germany, there is a fire in Werner Heisenberg's laboratory.

The 'Gunnerside' group parachute out of an aircraft over snow-covered Norwegian mountains and must trek to join up with the 'Grouse' team. Werner Heisenberg is back in favour with the German military.

Werner Heisenberg must continue his research without heavy water. When they learn that the Germans are rebuilding the factory, the Americans decide to bomb it, against Leif Tronstad's advice.

The Allies order the remaining members of the 'Gunnerside' team to sink the ferry carrying heavy water facilities across the lake from Rjukan. Kurt Diebner shows Werner Heisenberg secret documents.