Captain Rider Garforth is traveling on the liner Imperator to New York in 1920, with the intention of visiting a renowned doctor in an attempt to be cured of his shell-shock. The first-class passenger list is sparse but inhabited by lords and ladies, the rich, a smattering of "bright young things," respected professionals, and ex-soldiers. However, the bonhomie of the journey is shattered when a series of murders strikes terror into the hearts of the assembled cast. Rider, a former police inspector, is reluctantly coerced into investigating the murders, against the wishes of his trusted servant, Martins, who fears that the strain will be too much for the shell-shocked captain. As the pair investigates the mystery, Rider becomes embroiled with a beautiful woman and soon comes to realize that, in order to solve the murders, he must pay a painful, personal price...