Under the threshold of an old dilapidated house, builders discovered the bones of a murdered child. Is this the work of a sadist? Or an ancient, cruel ritual of sacrifice? Detective Harry Nelson, leading the investigation, again turns for help to the famous archaeologist Ruth Galloway, an expert on the religious cults of the past. However, so far their only clue is the story of the rector of a Catholic orphanage that was located in the house many years ago. Once upon a time, two children disappeared from this orphanage under mysterious circumstances - their traces could not be found. Perhaps the remains of one of these children were buried under the threshold? But then - who is the killer?