Complex sentence: The French told the Norwegian Government of the possible military significance of heavy water, who then gave the entire stock to a French Secret Service agent who smuggled it into France via England just before Germany invaded Norway in April 1940.
Split into simpler sentences: The French told the Norwegian government of the possible military significance of heavy water. Norway then gave the entire stock to a French Secret Service agent, who secretly brought it to France via England, just before Germany invaded Norway in April 1940.