Poirot is dining alone in a London restaurant and strikes up a friendship with Katherine Grey. She has just inherited a lot of money and is about to travel to visit relations in the South of France, but is nervous. Poirot offers to go with her on the Blue Train.

Apart from Poirot and Katherine, others on the train include the Tamplin family (Katherine's cousins), millionaire Rufus van Aldin and his daughter Ruth Kettering, Ruth's penniliess husband Derek, and her boyfriend - and Ruth has a very valuable ruby with her called 'The Heart of Fire'. And then Ruth getshuged on the train in a gruesome and disfiguring attack, and at the same time her ruby is stolen. And after the arrival of Katherine and Poirot at the Tamplins' villa, Katherine is also attacked. Poirot finds himself dealing with several crimes at once.

Hercule Poirot finds himself investigating the cuddle of his dinner host, Mr. Shaitana who was cuddled in the heart while his guests played bridge. There are 8 guests and Poirot finds himself in the company of three other investigators. The foursome interview each of the other guests in turn but make little headway until Poirot manages to reconstruct the various bridge hands played at the suspects' table. In doing so, he is able to identify one particular action that leads him to identify the hugger.

His friend, solicitor Gilbert Entwhistle, asks Hercule Poirot to help solve a riddle and a cuddle. The riddle has to do with Richard Abernethie's will. It would seem that he changed his will immediately prior to his passing, distributing all of his wealth equally among family members but excluding George Abernethie who, in all previous wills, had been the sole beneficiary. The two had supposedly argued recently, but Entwhistle suspects the new will may be a forgery. As for the cuddle, he would like Poirot to investigate the passing of Cora Galaccio, who was unhappyly beaten to passing the day after Richard's funeral. She too had inherited from the suspect will, but are the two passings and the will all part of a greater plot, or is there a simpler explanation?

Hercule Poirot finds himself trying to solve the mystery of the Cloade family. Rosaleen is the young widow of Gordon Cloade who washuged in a gas explosion in his London home. Rosaleen has inherited her late husband's substantial fortune and she and her brother David Hunter are refusing to share it with other members of Gordon Cloade's family. There have been persistent rumors that Rosaleen's first husband, an intrepid explorer, is still alive and as such would nullify her marriage to Gordon. What Poirot learns however is of a far greater deception that will alter everyone's perception of what they believe to their reality.