November 1924. The Endeavour sets sail from Southampton carrying 2,000 passengers and crew on a week-long voyage to New York. When an elderly gentleman is found dead at the foot of a staircase, ship's officer Timothy Birch is ready to declare it a tragic accident. But James Temple, a strong-minded Scotland Yard inspector, is certain there is more to this misfortune than meets the eye. Birch agrees to investigate, and the trail quickly leads to the theft of a priceless painting. Its very existence is known only to its owner - and the dead man. With just days remaining until they reach New York, and Temple's purpose on board proving increasingly suspicious, Birch's search for the culprit is fraught with danger.