First World War espionage was a fascinating and dangerous affair, spawning widespread paranoia in its clandestine wake. The hysteria of the age, stoked by those within the British establishment who sought to manipulate popular panic, meant there was no shortage of suspects. As head of Scotland Yard's Criminal Investigation Department, Basil Thomson was responsible for hunting, arresting and interrogating possible spies identified by the nascent British intelligence services before the First World War.