The safe exchange of wounded or gravely ill prisoners of war, 'protected personnel' – medical staff and clergy – and diplomats, civilians and alien internees is a little known dimension of the Second World War, yet it was highly dangerous work. Here, David L. Williams tells the gripping story of some fifty mercy ships engaged in these repatriation voyages, each of the exchanges arranged individually between Allied nations and the Axis belligerents, through neutral intermediaries, and often conducted under the supervision of the International Red Cross. Sailing alone and undefended through hostile waters, and conspicuously illuminated at night, the ships were constantly in danger from submarine and aircraft, their safety depending totally on the transmission and receipt of 'safe passage' commands to the armed units in their paths. However, despite the risk of attack and severe loss of life, these exchange operations were essential for providing a lifeline to thousands of people caught up in a cruel and brutal war.