This slow-paced mystery is set in early twentieth century England. The protagonist is Inspector Ian Rutledge of Scotland Yard, a man haunted by the ghost of a soldier named Hamish MacLeod, whose voice is his constant companion, conscience and advisor within his head.
The story begins with a group of schoolboys experimenting with alchemy by moonlight in the ruins of an abandoned Abbey. To their horror, they discover that they have apparently raised the devil himself, and swearing each other to secrecy, they run off into the night. The next morning, the body of an unidentified man is discovered in the ruins, dressed in a hooded cloak and gas mask, and next to his foot is a book on alchemy, property of the schoolmaster Albert Crowell.
Thus begins a long investigation into the identity of the dead man, the interrogation of the schoolmaster as a murder suspect, a couple of false trails, and the uncovering of a big cover-up by the British War Office. Along the way, sub-stories relate the circumstances leading to the death of Hamish and also the love life of the Inspector's sister Frances.
The trail takes Rutledge to a group of tiny houses in Berkshire, his job being to observe a man named Gaylord Partridge. The tourist attraction in the area is a huge figure of a horse, cut into the chalk in prehistoric times, and preserved in perpetuity galloping tirelessly along the hillside. Under the pretext of doing some horsing around on the cliffs, Rutledge learns that Partridge has disappeared, as he has been known to do on occasion, and that the occupants of the cottages all have secrets they'd rather keep hidden.
Amidst conflicts with the War Office, his own office politics and local law enforcement, Rutledge painstakingly pecks away at the armor of the residents of the Tomlin Cottages, and things start heating up both literally and figuratively when arson and murder go hand in hand.
A solid read, except for a few questionable plot contrivances, and packed with local color, this story starts off on a high note, and hastens to increase the pace as it wraps up at the end, but dallies too long in the middle for short attention spans.