Though I never read the book, personally I feel that this is an overly criticized and highly under rated movie. It's been awhile since I saw it so forget all the plot intricacies, but do recall enjoying this wartime romance at the time. As others have noted, it is definitely more a love story than a spy thriller. Also, I was sufficiently impressed with the performances by all three leads but especially Liam Leeson.
The plot revolves around an American spy master, Ed Leland, who reluctantly sends his secretary, Linda Voss, on a dangerous mission to Berlin, refusing to admit to himself that he has feelings for her personally. Voss is sent to spy on a high level German army officer named Dietrich, being deemed suitable for the task because she speaks fluent German. This secretary is half Jewish herself, and part of her motivation is actually to find some of her relatives who are in hiding. She thus goes undercover into Nazi Germany as a secretary turned spy, with little training in survival or espionage. However, she is a great fan of old spy movies and has gleaned a few tips! Perhaps the plot is not that plausible, but the same charge could be laid against any number of other movies. This is mainly a romance and a highly entertaining one.
Melanie Griffith is charming here in the role of Linda Voss. She is not a dumb blonde bimbo, nor does she even appear to be. Quite the contrary. Some have complained about her German, but I was interested in the story, and no expert on her accent. Michael Douglas competently portrays the American spy, Leland. Whether or not it is believable that someone in his position cannot speak a word of German, he has a very credible chemistry with Griffith. Liam Neeson is especially excellent, playing with dignity and charisma the complex role of the sensitive but potentially ruthless Nazi officer and devoted family man. Sir John Gielgud, the ultimate professional, is masterful as always and commands any scene in which he appears.
I confess that I'm a sucker for wartime romances, love the drama of that era as well as the 1940's styles, so would lap up just about any representative of the genre. While this one may not be a classic, it is a good love story with some gripping action, tension, and suspense. As another also noted, though it may indeed be highly improbable, it is nevertheless highly watchable.