November, 1940. Tom Tyler, Detective Inspector of the small Shropshire town of Whitchurch, is a troubled man. Only months earlier, he lost his soldier son, not in combat, but to a murderous enemy spy. His marriage is over, and the love of his life is far away, doing dangerous undercover work for MI6. So he jumps at the opportunity to help out in the nearby city of Birmingham, where an explosion in a munitions factory has killed or badly injured several of the young women who have taken on dangerous work in support of the war effort. At first, it seems more than likely the explosion was an accident, and Tom has only been called in because the forces are stretched thin. But as he talks to the employees of the factory, inner divisions - between the owner and his employees, between unionists and workers who fear communist infiltration - begin to appear.