November 1924. The Endeavour sets sail to New York with 2,000 passengers - and a killer - on board. 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 even Temple's purpose on board the Endeavour proving increasingly suspicious, Birch's search for the culprit is fraught with danger. And all the while, the passengers continue to roam the ship with a killer in their midst.