The Mystery of the Blue Train by Agatha Christie is a classic Hercule Poirot mystery involving a murder aboard the luxurious Blue Train traveling from London to the French Riviera.

Wealthy heiress Ruth Kettering is found dead in her compartment, her priceless ruby stolen. Hercule Poirot, who happens to be on the same train, investigates the case. As he unravels a web of jealousy, deceit, and hidden identities, he ultimately reveals the surprising murderer through his signature method of careful observation and deduction.