Poirot is asked by a Mr Jesmond, who is acting as an intermediary to an eastern prince, to help that unfortunate young man with a problem he is having. The prince is due soon to be married to a cousin and he has been enjoying his final days of freedom with a dubious young woman in London. The prince brought several expensive jewels with him to London for resetting by Cartiers and one of them, a fabulous ruby, was stolen by the young woman. If it cannot be retrieved a scandal will ensue and because of this the police cannot be involved. The mystery can be solved at an old English country house called Kings Lacey where it will be arranged for Poirot to join a family there for their Christmas celebrations, supposedly to experience a typical English Christmas.