The luxurious Transcontinental Express, a locomotive journeying coast-to-coast across the United States, is the height of 1930s technology and sophistication. Alongside carriages for dining, drinking and dancing, there’s even a swimming pool car.

And it’s there, just one day into the journey, that the corpse of a banker is found. With a killer on the train, the passengers soon begin to turn on one another. It's up to the clever Dr Pons – with his sharp wit and knack for impossible puzzles – to get to the bottom of the case, even as every new piece of evidence seems to suggest more perplexing possibilities.