Charles Lenox, Member of Parliament, sets sail on a clandestine mission for the government. When an officer is savagely murdered, however, Lenox is drawn toward his old profession, determined to capture another killer.

1873 is a perilous time in the relationship between France and England. When a string of English spies is found dead on French soil, the threat of all-out war prompts government officials to ask Charles Lenox to visit the newly-dug Suez Canal on a secret mission.

Once he is on board the Lucy, however, Lenox finds himself using not his new skills of diplomacy but his old ones: the ship's second lieutenant is found dead on the voyage's first night, his body cruelly abused. The ship's captain begs the temporarily retired detective to join in the hunt for a criminal. Lenox finds the trail, but in the claustrophobic atmosphere on board, where nobody can come or go and everyone is a suspect, he has to race against the next crime - and also hope he won't be the victim.