Horizon reveals the latest research into one of the most mysterious and common human experiences - pain. Breakthroughs have come from studying a remarkable woman in London who has felt no pain at all in her life, a man in the US who cut off his own arm to survive, and three generations of an Italian family who don't feel extremes of temperature. We witness a new treatment that involves a pioneering computer game 'snow world' that contains the power to banish pain. And we find how powerfully our moods and emotions shape what pain we feel.