When John Wilkes Booth fired his derringer point-blank into President Abraham Lincoln's head, he set in motion a series of dramatic consequences that would upend the lives of ordinary Washingtonians and Americans alike. In a split second, the story of a nation was changed. During the hours that followed, America's future would hinge on what happened in a cramped back bedroom at Petersen's Boardinghouse, directly across the street from Ford's Theatre. There, a twenty-three-year-old surgeon -- fresh out of medical school -- struggled to keep the president alive while Mary Todd Lincoln moaned...