An ancient curse echoes down the centuries and lays its shadow across the household at High Perwyl, the Welsh Border Castle where Sir John Meredith is trying to regain both physical and spiritual health after long and terrible war service coupled with cruel personal loss.

Why does young Sir Martin Lauder-Treaves fly into a rage merely because his charming American wife, Finella-Lou, drinks accidentally from his glass? What is the origin of the great marauding dog - if dog it is - that roams the countryside about High Perwyl at night?

As these and other sinister questions become increasingly urgent, the old spirit comes to life again in Sir John and he rouses himself to track down and destroy the evil thing that is poisoning the lives of his friends and hosts.