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