There is an enduring fascination with the secret history of the two world wars. This follow-up to Castaways of the Kriegsmarine examines the genesis of prisoner interrogation as an intelligence resource. We see how British naval intelligence officers were the first in the world to notice and exploit a loophole in the Hague Convention. We watch them develop a structured, systematic information source that produced important results in the First World War and made a critical difference to the second. We learn how the team's successes saved countless naval and merchant seamen's lives - and how they didn't get everything right. Some of the characters who created the service pop up repeatedly. Transcripts of prisoners' unguarded conversations reveal their individuality, and their struggle to reconcile conflicting loyalties. We think about the pressures that could induce a U-boat crewman to change sides. We consider the everyday serviceman's awareness and experience of the Nazi regime's atrocities. Two questions run through the book: 'Did it work?' and 'Did we use torture?' The answer to the first is an emphatic yes, to the second a qualified no.