Genevieve Masefield and George Porter Dillman have met all kinds of people while sailing as ship detectives for the Cunard Line. But as they prepare to embark on yet another Atlantic crossing, this time aboard the famous Caronia, they encounter an entirely new circumstance: two men brandishing shotguns herd a man and a woman aboard ship in shackles, right in front of scores of baffled passengers. Curious, the two detectives come to find out that the young couple have been captured in New York by Scotland Yard and are being transported back to England to stand trial for murder. Before long, Dillman and Masefield have a chance to talk to the Scotland Yard officers, and come away far less than convinced that the captured couple are the vicious criminals they're thought to be.