With the war in Europe winding down in the spring of 1945, the United States turns its vast military resources toward a furious assault on the last great stepping-stone to Japan—the heavily fortified island of Okinawa. The three-month battle in the Pacific theater will feature some of the most vicious combat of the entire Second World War, as American troops confront an enemy that would rather be slaughtered than experience the shame of surrender. Meanwhile, stateside, a different kind of campaign is being waged in secret: the development of a weapon so powerful, not even the scientists who build it know just what they are about to unleash. Colonel Paul Tibbets, one of the finest bomber pilots in the U.S. Army Air Corps, is selected to lead the mission to drop the horrific new weapon on a Japanese city. As President Harry S Truman mulls his options and Japanese physician Okiro Hamishita cares for patients at a clinic near Hiroshima, citizens on the home front await the day of reckoning that everyone knows is coming.