New York, 1915. RMS Lusitania, one of the world's most luxurious ocean liners, departs for war-torn Europe. Among those on board is Patrick Gallagher, a civil servant in Her Majesty's government tasked with escorting a British diplomat back to England. When a fellow passenger is believed to have shot himself in his cabin, Gallagher is asked by the captain to investigate the scene, but one crucial detail doesn't fit. The man's body was discovered in a locked cabin with the key inside and no gun to be found. Was it really death by suicide? Or murder? Gallagher believes one of the passengers is a deadly killer who could strike again to protect their true reasons for being on board. And all the while, the ship sails on towards Europe, where deadly submarines patrol dark waters.