The story is set in the scenic North Downs of Surrey, on the ridge known as the Hog's Back, at the time of the building of the A31 bypass.

Ursula discovers that not only is Julia having an affair with her neighbour Reggie Slade, but that Dr. Earle has been seen in London with a mysterious woman, dressed in grey.

As tensions mount, Dr. Earle disappears from his study in extraordinary circumstances: one minute he is sitting in his living room, comfortably settled with newspaper and slippers, and the next he has vanished.

The local police finally turn to Inspector French of Scotland Yard to help solve the mystery.

The case soon takes a more complex turn; other people vanish mysteriously, including one of Dr. Earle's house guests.