The highlights of the book are here. However different people are get credit the different actions and thoughts than in the book.
The first half of the movie is about solving a problem of how to make the Germans think that the invasion will take place in Sicily. The solution is barbaric and unique. The second half of the movie is to keep the Germans from finding out the deception.
One of the most telling scenes is while there are preparing the deception and are in a bunker you hear the bombs and screams that remind you of the urgency and purpose of the movie.
Clifton Webb is Lieutenant Commander Ewen Montagu, the person that the movie evolves around. Gloria Grahame plays Lucy Sherwood, an alleged girlfriend. The reason she shines is that they put a lot of grease on her face and it shines in the dark.
If you saw Ben-Hur then you may recognize Messala (Steven Boyd) as the spy sent to be sure that Martin is genuine.
And if you are an Agatha Christie Miss Marple fan you will see that Joan Hickson is the apartment owner.
It is a great story and well told. It makes you want to find shirts with separate collars.