Their friend Father Higgins is in trouble, and it is up to Johnny and Professor Childermass to save him from the forces of evil. In this, the eighth Johnny Dixon mystery, Johnny and the professor hear word of Father Higgins being haunted, and when he disappears, clues lead them to England. There they find the priest has been possessed by Dr. Masterman, the ghost of a deranged knight bent on ruling the world. Masterman intends to reanimate the bodies of seven evil fellow knights trapped in a hidden, underground room. Hundreds of years ago, the knights sold their souls to the devil and, if Masterman can bring them back, the entire world would be his to control. Can Johnny and the professor stop him in time? Is there any hope of getting their friend back from the spirit's clutches?