Rudi Graf used to dream of sending a rocket to the moon. Instead, he has helped to create the world's most sophisticated weapon: the V2 ballistic missile, capable of delivering a one-ton warhead at three times the speed of sound. In a desperate gamble to avoid defeat in the winter of 1944, Hitler orders ten thousand to be built. Graf is tasked with firing these lethal 'vengeance weapons' at London. Kay Caton-Walsh is an officer in the Women's Auxiliary Air Force who joins a unit of WAAFs on a mission to newly liberated Belgium. Armed with little more than a slide rule and a few equations, Kay and her colleagues will attempt to locate and destroy the launch sites. As the death toll soars, Graf and Kay fight their grim, invisible war - until one final explosion of violence causes their destinies to collide...