The synopsis is elsewhere but given that this film is based on the memoirs of the real-life man who also stars in the film he was a very brave man to take on the role as Montgomery and he must have been a good actor to try and pick up M's habits, gait and mannerisms to the level that it fooled the Germans. Clifton James in this film probably did just what he did in the 'real' scenario but it does not detract from what he did that in the end probably saved a lot of lives. If you read the WIkipedia page on him, the links with well-known people come out that I didn't know - q.v. David Niven, Dennis Wheatley and the fact that James had to have a prosthetic finger
Shaggy dog World War II tale based on an actual incident of a soldier promoted to impersonate Field Marshall Montgomery in the run up to D Day. The intention was to fool German intelligence about where the landings would occur. The film supposes his imitation was so perfect, the Nazis tried to have him killed, though the events are embellished.
The amazing concept of the production is that Monty's double is played by the real life imitator! Based on his own bestseller. This is ME Clifton James who re-enacts history before our eyes. The drawback is that he is quite a wooden actor, though with an amazing likeness for the leader of British army operations in WWII.
The film is pitched as a comedy, with a more serious climax where the Germans attempt to kidnap the fake general. Which never really happened. It's mostly a two hander between James and John Mills in jovial form as the actor's personal coach from British intelligence. Which in reality was the task of David Niven!
It's a likeable, lightweight caper, with little impression of the real risks that James took when plucked out of the pay corps to lead the deception. It comes towards the end of the cycle of fifties WWII films based on the experiences of those who served, often in very unusual and imaginative ways. None more strange than the story of Lieutenant James.