Big Brother's Little Test

The ' polygraph' lie-detector has arrived in Britain. It measures tiny changes in your breathing, perspiration, and heartbeat, as they ask you questions. Someone's hurt - did you do it? Did you steal money from the till? Did you lie when applying for your job? Q.E.D. observes the polygraph in action in America, sees a British ex-policeman in training in Atlanta, and meets a man found guilty of murder after a polygraph test. In Britain, we may soon rely on it for our national security. But is that wise? How does the polygraph work? How effective is it? Can you beat it? And who's next for the little test - could it be you?