Norwegian resistance fighters try to stop Nazi experiments in nuclear warfare.

In Nazi-occupied Norway in 1942, underground leader Knut Straud smuggles a microfilm message from an agent in a Nazi factory hidden deep in a Norwegian valley to university Prof. Rolf Pedersen. The Nazis are producing "heavy water," an essential in the manufacture of atomic bombs, at the factory. Straud and Pedersen hijack a boat and bring the message to the Allies in England, who decide to destroy the factory. Straud and Pedersen are parachuted, along with a team of assistants, back into Norway to lay the groundwork for an attack on the factory by British commandos. The gliders carrying the commandos crash and burn, whereupon the underground team decides to try to destroy by themselves the room at the factory containing the "heavy water" machinery. They succeed, but Pedersen is subsequently wounded and captured. He escapes and rejoins Straud. The factory is back in operation, however, when the Nazis ship in duplicates of the demolished machinery. Allied bombers attack the factory but are unable to hit the high-concentration room, and the Nazis, their position discovered, decide to ship the "heavy water" to Germany under tight security. Rolf, after saving the lives of most of the innocents aboard, sinks the ferry carrying the "heavy water." The cargo is forever lost to the Nazis.