During the Second World War, the Nazis took many art treasures for "care and safe-keeping", including the priceless collection of French Impressionist paintings built up by the industrialist Friedrich Carl Siemens in 1930s Berlin - among them works by Manet, Monet, Degas and Cezanne. At the end of the war, when the Americans and the Russians reached Berlin, these treasures went missing and have not been seen since. Timewatch goes in search of the missing treasures and unravels an extraordinary story of official looting.