As head of Scotland Yard's CID, and working in tandem with MI5, Basil Thomson was in charge of hunting down German spies during the First World War and uncovering communist subversives in the trades unions in the three years following the war. As well as apprehending, he was responsible for interrogating captured agents, among them Mata Hari and Roger Casement. In this book, originally published in 1922 as 'Queer People', he gives an account of his extraordinarily exciting work.