I Was Montys Double (1958) - John Mills

Is a semi-fictional account of a real life WWII event. It's the spring of 1944. Major Harvey, with the British Army's Secret Service, he just returned to London after a successful mission on the continent, is looking forward to his scheduled break. However, his superior, Colonel Logan, informs him that his leave has been canceled in he tasked with another time sensitive mission. The Germans are aware that British troops will this year land on the continent, but they don't know when or exactly where, although the highest probability is that they will just cross the English Channel to land somewhere on the northwest French coast. What Logan's superiors want them to embark on is a misinformation campaign as to the Allied landing intentions, the primary goal being that misinformation making its way to Berlin. Harvey doesn't think that they can credibly fool the Germans no matter what misinformation campaign they devise, that is until Harvey attends a music hall show, the patriotic revue which includes someone coming onto the stage to raucous applause, his name not even need to be mentioned: General Bernard L. Montgomery. Being a revue, the person obviously is not "Monty" (how he is commonly referred) but an actor who even Harvey could admit, at least from a distance, looked like Monty. Harvey learns the man is Lt. M.E. Clifton James, a professional actor who could easily pass for Monty. The plan would have James pretend to be Monty, the most likely person to lead the advancing troops, and have him stationed in the eastern Mediterranean far away from where the real Monty would lead the troops onto the continent. There are many potential problems to the plan, including convincing James to this "acting job" different from any acting job he's ever had and have him confident enough to pull it off in its inherent dangers, and in turn convince those he would encounter, who may include people who have actually met the real Monty, that he is who he says he is and that he is in the area to lead the Allied troops north onto the continent.