The Mystery of the Blue Train revolves around a set of fabulous rubies, said to have been worn by Catherine the Great, now purchased by an American millionaire as a gift to his daughter. When she takes them with her on a luxury train to the Riviera, she is unaware that among her fellow passengers are her unfaithful husband, his not-so-secret mistress, a quiet Englishwoman who believes that nothing exciting ever happens to her—and the famous retired detective Hercule Poirot. And, possibly, a murderer.

Despite positive critical reception of this entry in the Poirot canon, Christie described this as the least favorite of her own novels. However, this is almost certainly due to the events in her personal life during that time: the death of her mother, followed closely by the discovery of her husband’s affair and his request for divorce. Suffering from what appears to have been a nervous breakdown, Christie famously disappeared for over a week in December 1926, triggering a nationwide search before she was discovered to have checked into a hotel under an assumed name. This novel was written in part during a three-month convalescence following those events.

The book is dedicated to Christie’s secretary Charlotte Fisher and her pet terrier Peter, calling them “distinguished members of the O.F.D.” The “Order of Faithful Dogs” was Christie’s name for her circle of closest friends.