John Buchan (1875 - 1940) was a scottish writer, barrister, Member of Parliament and later become Baron Tweedsmuir and Govenor-General of Canada. While he was most famous for adventure stories, such as The Thirty-nine Steps, he was also popular for his supernatural horror stories, of which The Watcher by the Threshold is a prime example.

When Harry Grey receives a mysterious summons from his cousin Sybil to visit her and her husband Robert Ladlaw at their remote scottish estate, he is taken aback by the urgency of the postscript. "For Heaven's sake come and see us!" she writes. "Bob is terribly ill, and I am crazy. Come at once." And then to finish: "Don't bother about bringing doctors. It is not their business."When Grey undertakes the dismal journey to the House of More where the Ladlaws live, he finds a situation far more mysterious and terrifying than he could ever have imagined.

Ladlaw is in the grips of a terrible possession... and Sybil is at her wits end. Ladlaw's split personality is so bizarre that it is sometimes hard to grasp who has the upper hand.