"The Mystery of the Blue Train" is a detective novel written by Agatha Christie, first published in 1928. It features Christie's famous Belgian detective, Hercule Poirot, who investigates a murder that occurs aboard the luxurious Blue Train traveling from Calais to Nice.

The story revolves around the murder of Ruth Kettering, a wealthy American heiress, whose valuable ruby necklace is also stolen. As Poirot delves into the case, he encounters a cast of characters with motives ranging from financial gain to personal vendettas. Suspects include Ruth's estranged husband, her lover, and other passengers aboard the train.

As Poirot unravels the complexities of the case, he employs his keen powers of observation and deduction to uncover the truth behind the murder and the missing jewels. "The Mystery of the Blue Train" is celebrated for its intricate plot, memorable characters, and surprising twists, all hallmarks of Agatha Christie's masterful storytelling.