Karl Hausmann, a Nazi war criminal and explosives expert, is released from prison and vows revenge on the four judges who sentenced him, sending them each a letter threatening them. The first, an American, is killed by a remote-control bomb in New York; in Paris he is able to pass a booby trap to his victim's wife which she unwittingly takes into their apartment in spite of Interpol's precautions.. Duval travels to London where the last two victims, from Britain and Russia, are due to attend a conference; knowing it is the ideal opportunity for Hausmann, he hopes he can spot any attempt on their lives before it is too late..