On May 1, 1915, a luxury ocean liner as richly appointed as an English country house sailed out of New York, bound for Liverpool, carrying a record number of children and infants. The passengers were anxious. Germany had declared the seas around Britain to be a war zone, and for months, its U-boats had brought terror to the North Atlantic. But the Lusitania was one of the era's great transatlantic "Greyhounds" and her captain, William Thomas Turner, placed tremendous faith in the gentlemanly strictures of warfare that for a century had kept civilian ships safe from attack. He knew, moreover, that his ship -- the fastest then in service -- could outrun any threat. Germany, however, was determined to change the rules of the game, and Walther Schwieger, the captain of Unterseeboot-20, was happy to oblige. Meanwhile, an ultra-secret British intelligence unit tracked Schwieger's U-boat, but told no one. As U-20 and the Lusitania made their way toward Liverpool, an array of forces both grand and achingly small -- hubris, a chance fog, a closely guarded secret, and more -- all converged to produce one of the great disasters of history.