Robert and Katherine Thorn seem to have it all. They are happily married and he is the US Ambassador to Great Britain, but they want more than to have children. When Katharine has a stillborn child, Robert is approached by a priest at the hospital who suggests that they take a healthy newborn whose mother has just died in childbirth. Without telling his wife he agrees. After relocating to London, strange events - and the ominous warnings of a priest - lead him to believe that the child he took from that Italian hospital is evil incarnate.

Simon Cordier is a well-respected magistrate who visits a condemned prisoner, Louis Girot, just before the man's execution. Girot again pleads his innocence insisting that he has been taken over by a spirit that forced him to commit his crimes. Cordier doesn't believe him and the man suddenly dies. Cordier does however note a rapid change in his personality during their short interview. In the following days, Cordier must face a number of strange occurrences in his home. He begins to wonder if he is sleepwalking but is soon hearing voices and begins to wonder about his sanity. It's recommended to him that he take up sculpting, something he once had an interest in. He develops a relationship with Odette, a gold digger married to a struggling artist, but the evil, invisible spirit soon drives him to murder.