This movie begins with a frighteningly horrible sequence in which Captain Wiesler questions a hapless young man suspected of helping a friend escape from the former German Democratic Republic (GDR) to the west.
The ruthless grilling is intercut with his lecture to new Stasi recruits in which this interrogation has become a set-text: the man's pleas, screams and sobs are replayed on tape to the earnest young students. There is a particularly queasy detail concerning the vital importance of making all terrified suspects keep their sweaty palms pressed to the seat of their chair: it is so that a sample of sweat can be kept on an impregnated cloth, in case it has to be given to the tracker dogs.
This fierce and gloomy drama is an indictment of the sinister brutalities of the Stasi, the GDR's secret police, whose tentacular network of informers was so vast that fully 2% of the entire civilian population was on the payroll - a network of fear and shame worthy of George Orwell's Nineteen Eighty-Four.
It's a frightening movie, more so because it's true in every detail, including when an incautious Stasi official is shown telling a joke in the staff canteen about Erich Honecker: the party chairman, the joke goes, greets the sun with a cheerful "Good morning!", to which the sun replies obediently at various stages of the day, except when it sets in the evening, when it jeers: "Screw you, I'm in the west now!" Horribly, this party official is later shown reduced to humiliatingly minor clerical duties, presumably because of this one, ill-timed joke.
On a personal level, it's even more frightening to think that it might've been me telling that joke if we hadn't left for the west in 1949. Chilling and powerful. Yes -- it's in German (with English subtitles). Get over it.
P.S. Read more about the plot on Wikipedia.