Now this is a story that deserves to be as well known as the legendary activities of the Norwegian commandos and British bombers that destroyed the heavy water production capabilities of the German nuclear program.
These actions explain why the Germans were unable to make use of Norwegian heavy water after 1943 but of course the plant had been open for nearly a decade before that. One of the reasons why the Germans didn't find a nice cache of heavy water waiting for them on the occupation of Norway is that the French government had already obtained the entire world stock, approximately 40 gallons, on the instructions of Lew Kowarski and Hans von Halban.
In the chaos following the fall of France and amidst the installation of the Petain government the two scientists smuggled all of the heavy water by automobile and British ship to the United Kingdom where it could be commandeered by the Allies. For a characterful and idiosyncratic account of this adventure read sections II and III of this interview with Kowarski in 1969:
... and just in case that doesn't grab your attention the captain of the boat that recovered the heavy water was a wonderfully piratical, bomb-diffusing Earl of Suffolk.