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.
On his way to be married, he met an attractive but obviously dubious young woman, and rashly decided to have a last fling with her. The prince brought several expensive jewels with him to London for resetting by 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.
On Christmas Eve at Kings Lacey, Poirot is told about the other members of their party by Mrs Lacey, his elderly host in the house. Joining them will be her husband, Colonel Lacey; Sarah, a granddaughter by their deceased son; Colin, a teenage grandson by their daughter; Michael, a friend of Colin's at school; Bridget who is of the same age as Colin and Michael and is a great niece of Mrs Lacey's; Diana, a young cousin of Mrs Lacey's; and David Welwyn who is a family friend.
Colonel and Mrs Lacey are perturbed by Sarah's relationship with a young rake called Desmond Lee-Wortley. They both think him unsuitable for their granddaughter and have invited him to join them for Christmas in the hope that a few days' close contact with Sarah will show her how unsuitable he is, particularly in contrast to David Welwyn who has been friends with Sarah since childhood. Lee-Wortley is there with his sister who is recovering from an operation and is confined to her room, convalescing.
Colin, Michael, and Bridget are disappointed with Poirot as he does not meet their expectations of what a detective should look like. They hatch a plan to arrange a false murder for Poirot to detect with Bridget lying in the snow with blood as the "dead" body and footprints leading through the snow which is now falling and expected to grow heavier. They decide that they will put their plan into operation on Boxing Day as Colonel Lacey would not like something of that nature to take place on Christmas Day itself.
That night, the Christmas tree is decorated and the party retire to their rooms for the night. On his pillow, Poirot finds a scrawled note which reads, "DON'T EAT NONE OF THE PLUM PUDDING. ONE WHO WISHES YOU WELL". He is most puzzled.
The next day, the party eats a huge Christmas dinner and then the elderly and partly retired butler brings in the Christmas pudding with great ceremony. The diners find the usual tokens in their portions but the Colonel is annoyed and amazed when he almost chokes on a piece of red glass in his. Poirot takes the object and pockets it. Afterwards Poirot visits the kitchen to compliment the daily cook, Mrs Ross, on the meal and particularly the pudding. She confesses that two were made, one for today and one for New Year's Day but the one for today was dropped and the one for six days' time substituted in its place.
That night, Poirot pretends to sleep in his bed, having avoided drinking a drugged coffee which had been handed to him by Lee-Wortley. A figure enters his room and conducts a fruitless search.
The next morning, the children carry out their "murder" plan and rouse Poirot from his bed to investigate the "dead" body but the planners get a shock when Poirot confirms that Bridget is indeed dead. Sarah and Lee-Wortley having joined them in the snow. Poirot invites the young man to check Bridget's pulse and he confirms there isn't one.
Poirot points out that the footprints in the snow look like Lee-Wortley's and that in the dead girl's hand is the glass "ruby" from the pudding. Lee-Wortley is dumbfounded but, taking the glass, offers to ring for the police. Claiming to Diana that the phone is dead, he drives off to fetch them.
Poirot takes the others in the house where he explains all. He tells them that Lee-Wortley is a blackmailer and involved in other questionable matters. His supposed sister is the young woman who took the ruby from the eastern prince and the two of them were tracked to Kings Lacey.
Bridget appears in the room—she is not dead, having worn a tourniquet on her arm when lying in the snow—and she was in league with Poirot to trick Lee-Wortley. Poirot heard the children planning their "murder" through an open window and used this opportunity to take Lee-Wortley in. The "ruby" that Bridget held in the snow was a paste copy that Poirot brought with him to the house and Lee-Wortley has taken this with him. Poirot supposes that he will go abroad where he will be surprised when he tries to sell the false jewel. The real ruby was hidden by the two thieves in what they were told was the New Year pudding and they were unaware of the accident that befell the pudding intended for Christmas Day. Lee-Wortley's "sister" overhears this and is furious that her co-conspirator has left her to face the music. She too flees the house. The mystery of who left the note on Poirot's pillow is solved when one of the housemaids confesses that she heard Lee-Wortley and his "sister" discussing getting Poirot out of the way and that something had been put in the pudding, causing her to think they planned to poison him. He rewards her by promising her a vanity box, and gets a kiss from Bridget under the mistletoe.