What They Found, the first documentary by the film and theatre director Sam Mendes, is a short, stark shock. The film straightforwardly combines two precious artefacts held at the Imperial War Museum in London: 35mm film, shot by Sgt Mike Lewis and Sgt Bill Lawrie of the British Army Film and Photographic Unit, before and during the liberation of the Bergen-Belsen concentration camp near the town of Celle in northern Germany in April 1945; and audio interviews given by the cameramen in the 1980s. Lewis and Lawrie did not record sound when they visited Belsen; the words they spoke years...