Hercule Poirot is called in during the early part of 1923 to investigate the kidnapping of three-year old Johnnie Waverly, the son of Marcus Waverly from his old family home, Waverly Court in Surrey. For several days prior to the kidnapping, the family received anonymous letters which threatened to take the boy unless twenty-five thousand pounds was paid. Mr. Waverly took the letters to the police who took little interest until a final letter was received which stated that the boy would be kidnapped at twelve o'clock the next day. On the day that the threat was due to take place, Mrs. Waverly was mildly poisoned and a note was left on Mr. Waverly's pillow which stated, "At Twelve O'Clock". Horrified that there was someone inside the house who was involved, Mr. Waverly sacked all of the staff except for his long-time Butler, Tredwell and Miss Collins, his wife's trusted secretary-companion.
At the appointed time, Mr. Waverly, his son and Inspector McNeil of Scotland Yard were in a locked room in the house with police posted in the extensive grounds. Dead on twelve, the police found a tramp sneaking toward the house who had on him cotton wool, chloroform and a post-kidnap note ready to plant. Mr. Waverly and the Inspector dashed outside to see what was happening and the boy was taken then – driven off in a car through a now unguarded gate. The astonished people then heard the village clock chiming twelve and realised that the main clock in the house had been put forward by ten minutes. A car with a small child in it was stopped some time later but found not to be the one that the police were looking for. The tramp claims he was employed by Tredwell but the butler has an alibi for the time that he was supposedly meeting the tramp as he was in the house with Mr. Waverly at the time.
Poirot travels to Waverly Court and is told of the existence of a priest hole. In it, he finds the footprint of a small dog in one corner but no one knows of any such creature small enough in the house. After questioning the child's sacked nurse, Tredwell and Miss Collins, Poirot concludes his investigation and confronts Mr. Waverly. He 'kidnapped' his own son to get money from his rich but very careful wife. The poisoning of the wife to incapacitate her, the note on the pillow and the re-setting of the clock point to an inside job and only Mr. Waverly could sack all of the servants to reduce the level of protection around the child. Tredwell was in on the matter being a devoted servant of his master and he did indeed employ the tramp. The footprint of the 'dog' in the priest's hole was that from a toy kept there to amuse the boy until he could be spirited away afterwards. A shamefaced Mr. Waverly confesses to Poirot and reveals that the child is presently with his old nurse.