It starts with Vice President Harry Truman elevating to the Presidency with the death of Franklin D. Roosevelt on April 12, 1945 and covers Truman and others involved in the effort to build and then drop the two atomic bombs on Japan August 6 and 9.
In addition to giving details about the events, Wallace writes about the compelling people involved in them. Featured are Truman, Manhattan Project scientific director J. Robert Oppenheimer, military head General Leslie Graves, Colonel Paul Tibbets Jr. who piloted the plane that dropped the first bomb, one of the women working control dials in the uranium separation plant in Oak Ridge, TN, and a young girl in Japan. Wallace details how after being sworn into office, Truman was told by his Secretary of War that there was an urgent matter they had to speak about, a project underway to develop a new explosive of great power.
Along with the development of the bombs, there's also interesting material in the book about Truman wrestling with the decision of whether or not to drop them. Germany surrendered on May 8, 1945 and projections Truman received had the invasion of Japan by American troops costing the lives of an estimated 250,000 Americans. He had to balance that information against dropping weapons of such destructive power.
The test of the bomb was done on July 16, and the weapons-grade uranium for the bomb was delivered to the Far East about the USS Indianapolis, sunk days later by a Japanese torpedo. Tibbets and his flight crew flew the Enola Gay and the bomb codenamed Little Boy close to seven hours from Tinian Island to Hiroshima. It exploded 1,890 feet above the city, with the Enola Gay then six miles away, flying fast to try to get away from the blast and accompanying shock wave. On August 8, the Soviet Union declared war on Japan and the dropping a day later of the bomb Fat Man was a mission riddled with near misses. The plane, Bock's Car, was short on fuel, not enough to make it back to Tinian so landed on Okinawa. Also, the original target was the city of Kokura, but weather conditions there led to the alternate target of Nagasaki being bombed.
Japan surrendered shortly after and the epilogue of the book covers the lives after the war of people featured in the book, and how they viewed the dropping of the bombs and creation of the nuclear age.