For better or worse, Navy captain William S. "Deak" Parsons made the atomic bomb happen. As ordnance chief and associate director at Los Alamos, Parsons turned the scientists' nuclear creation into a practical weapon. As weaponeer, he completed the assembly of "Little Boy" during the flight to Hiroshima. As bomb commander, he approved the release of the bomb that forever changed the world. Yet over the past fifty, years only fragments of his story have appeared, in part because of his own self-effacement and the nation's demand for secrecy. Based on recently declassified Manhattan Project documents, including Parsons's logs and other untapped sources, the book offers an unvarnished account of this unsung hero and his involvement in some of the greatest scientific advances of the twentieth century.