On May 1, 1915, with WWI entering its tenth month, 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 surprisingly at ease, even though Germany had declared the seas around Britain to be a war zone. For months, German U-boats had brought terror to the North Atlantic. But the Lusitania was one of the era's great transatlantic "Greyhounds"--the fastest liner then in service--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. 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.