Shot in 1944 entirely in French (version seen has English subtitles), in style rather like a short section of a budget version of his "Lady Vanishes", it must have been intended for clandestine distribution in occupied and Vichy France. For this reason and the fact that it appears to have a clear practical purpose it presumably is a training film. It is thus not primarily a tribute to the French Resistance, its purpose seems instead to be to warn them of a dangerous new Gestapo method of infiltration. Hitchcock's contribution was to make the message clear and hold the attention.
It concerns the return to England of a British air-gunner (played by English actor John Blythe in French) recently escaped from a German prisoner of war camp. He has safely arrived back in London due to the help he received from the French Resistance and is being de-briefed by French exile officers. What should though have been a happy conclusion to a successful escape is soured when the true tragic facts are revealed to him that he has been an innocent pawn in a dastardly Gestapo design for the infiltration of the French resistance network. In this he was made a carrier of something deadly - not disease but something similarly poisonous - that he had been made to unwittingly expose members of the Resistance. One guesses that the main purpose of the film was to warn everyone of this.