In 1960s London, Charles Dobbs is a staid MI5 operative investigating Foreign and Commonwealth Office official Samuel Fennan, a former Communist who apparently commits suicide. Dobbs becomes suspicious about the cause of Fennan's death while visiting Fennan's widow the morning after his death. When a wake-up call is received at Fennan's home, his widow Elsa says the call was for her. Dobbs discovers this to be a lie, and as a result Dobbs suspects that Elsa, a survivor of a Nazi extermination camp, might have some clues regarding Fennan's death.