Behind the locked doors of three mansions in London’s exclusive Kensington Palace Gardens neighborhood, the British Secret Service established a highly secret prison in 1940: the London Cage. Here recalcitrant German prisoners of war were subjected to “special intelligence treatment.” The stakes were high: the war’s outcome could hinge on obtaining information German prisoners were determined to withhold. After the war, high-ranking Nazi war criminals were housed in the Cage, revamped as an important center for investigating German war crimes.