In August 1949 a newspaper article caught the eye of William Peter Blatty, then an undergraduate. It described many of the bizarre and harrowing events upon which Blatty would later base his bestselling novel, The Exorcist, and the film of the same name. 40 years later a priest's diary came to light. It had been hidden in a locked room in a St Louis mental hospital, and turned out to be a detailed account written by one of a team of Jesuits who had carried out a series of exorcisms of the boy at the centre of the case. The diary came into the hands of Thomas Allen, who researched its contents thoroughly, tracking down all surviving witnesses, including the victim. This book is the result of Allen's research. His other books include Wargames.