“Documentarians Charles Frend (director) and Alberto Cavalcanti (Associate Producer) here take a true story of a Welsh engineer who went to France at the beginning of the war to prevent intricate machinery from falling into the hands of the advancing Germans, and turn it into a film that works as a thrilling adventure story, quite apart from its wartime orientation, and also benefits from an underplayed documentarian approach. (We except Robert Morley, playing a Gestapo hireling, from that generalization!) Typical of the best of Ealing, with propaganda rearing its head only at the fadeout.” William K. Everson