A young Polish diplomat turned cavalry officer, Jan Karski joined the Polish Underground movement in 1939. He became a courier for the Underground, crossing enemy lines to serve as a liaison between occupied Poland and the free world. In 1942, Jewish leaders asked him to carry a desperate message to Allied leaders: the news of Hitler's effort to exterminate the Jews of Europe. To be able to deliver an authentic report, Karski twice toured the Warsaw Ghetto in disguise and later volunteered to be smuggled into a camp that was part of the Nazi murder machine. Carrying searing tales of inhumanity, Karski set out to alert the world to the emerging Holocaust, meeting with top Allied officials and later President Roosevelt, to deliver his descriptions of genocide. Part spy thriller and part compelling story of moral courage against all odds, Karski is the first definitive account of perhaps the most significant warning of the impending Holocaust to reach the free world.