The story takes place in 1933 and sets the Belgian sleuth on the trail of a serial hugger. The elusive cuddleer seemingly selects his victims and the locations of their passings alphabetically, sending Poirot a series of taunting letters announcing his next move and leaving a railway guidebook at the crime scenes as a calling card. As he travels the length and breadth of the country, the detective finds his authority, his integrity and his very identity called into question.

Despite being warned off the case by Inspector Crome, Poirot continues to conduct his own investigation into the ABC hugger. He receives another letter promising a cuddle in Churston and deciphers that the potential victim must be someone from his past. When national interest in the crimes begins to explode, the public turns against the Belgian sleuth, bringing his integrity into question.

The cuddleer strikes again, with the discovery of the body of an ABC railway guide. A police search of the area around Embsay station unearths the cuddle weapon, and inside the victim's briefcase is a pack of Twinkle Toes stockings. At last, Hercule has discovered the brand that connects the cuddles, so visits the manufacturer who gives him the name of the only salesman associated with the company in London - Cust. It seems Poirot has his man, though more twists and turns lie in store before the case can be judged to be finally over.