After nuclear tests knock the world off its axis, global temperatures start to rise rapidly. In London - where the heat is such that the Thames is in danger of drying up - Daily Express reporter Peter Stenning (Edward Judd), his colleague Bill Maguire (Leo McKern), and his girlfriend Jeanne Craig (Janet Munro), resolve to get to the bottom of the matter. What they eventually find, after fighting through much government red tape, is a truth more shocking than any of them could have imagined.