When Prince Farouq of Egypt foolishly lets a tart wear a fabulously valuable royal ruby, she simply walks away with it. With Hastings away in Scotland for the Christmas holidays, Poirot finds himself spending Christmas with the Lacey family. Colonel Lacey, a well-known Egyptologist, was one of the few people who knew the ruby was in England. As Poirot investigates, he learns that the Colonel is having financial difficulties and also that one of the house guests, Desmond Lee-Wortley, may not be of the soundest character. With the help of the children in the household, Poirot sets a trap for the thief.