The collapse of the Soviet Union has left thousands of nuclear warheads, vast amounts of plutonium and entire cities full of scientists who know how to make nuclear weapons: what happens to them now? How safe are the warheads? Where is all the plutonium? And could other countries entice the scientists to make their own atomic bombs and spread the threat of nuclear war around the world? David Dimbleby presents a debate on the "nuclear nightmare" with experts from the USA, Russia and the UK.