This chapter examines the functioning of MI5’s Camp 020 during the Second World War, mainly using the camp’s administrative files. It explains how most detainees were never brought to trial, among other reasons because of anxieties that legal proceedings might expose the camp’s existence. This reality turned Camp 020 into a central but secretive instrument of counter-espionage. Turning to the treatment of detainees, the chapter shows that psychological pressure was integral to the camp’s method, justified as a necessary means of asserting control over prisoners. Yet such practices were bounded by a self-imposed prohibition against physical coercion, a limit that was closely monitored to preserve the legitimacy of the operation and the reliability of its intelligence. Finally, the chapter argues that Camp 020’s interrogations did not simply replicate information from other sources: they clarified, contextualised and connected insights from across the intelligence system, from signals to captured documents. Together these findings explain how Camp 020’s practices and output contributed to MI5’s growing confidence that the German intelligence effort posed less of a threat than initially feared.