It is the spring of 1937 and distinguished economist Lord Benyon is on the Queen Mary, bound for New York harbour. His highly important mission is to persuade President Roosevelt to provide Britain with arms and money if it comes to war with Germany - as seems very likely. Many people want the mission to fail and will not stop at murder to achieve their aim. Major Ferguson of Special Branch and a friend of Lord Edward Corinth, asks him to keep an unofficial eye on Benyon who refuses to be surrounded by policemen on the Queen Mary but is prepared to have Edward at his side. Also on board is Verity Browne, going to America at the Party's behest to liaise with Communist sympathisers there.

But it is not Lord Benyon who is murdered but the racist senator from South Carolina who has managed to enrage a number of his fellow passengers - not least Warren Fairley, the black singer, actor and communist. But surely Fairley is too obvious a suspect? Might not the murderer be the German aeronautical engineer, or Marcus Fern, the city banker who is acting as Benyon's secretary? And what about Bernard Hunt, the art dealer, or even the young American union organiser Sam Forrest, with whom Verity is so taken?