April 12, 1945. After years of bloody conflict in Europe and the Pacific, America is stunned by the death of President Franklin Roosevelt. In an instant, Vice President Harry Truman, who had been sidelined from war planning and knew nothing of the top-secret Manhattan Project to develop the world's first atomic bomb, must assume command of a nation at war across multiple continents and make one of the most important decisions in history.

Countdown tells the gripping story of the turbulent months, weeks, and days leading up to August 6, 1945, when Truman gives the order to drop the bomb on Hiroshima.