Young and famous for his modern investigative methods detective Janos Marton (Jan Kolenik) from Pest comes to Betliar castle to solve the mysterious, brutal murder of a maid. At Gemer, he is greeted by the local guard Vilem Cerny (Stanislav Majer), who does not like the presence of an overly confident investigator from Pest in his district and the need to cooperate with him at all... and it seems that Marton's presence does not suit the local residents either, who are distrustful of technical innovations and prefer to believe in magic and superstition instead of science. However, it turns out that the murder of the maid is far from as simple as the investigators would like; and even that it is not the only strange death of a young woman in this area... So Marton and Cerny are faced with a difficult challenge - to overcome their mutual dislike and search together for the perverted perpetrator, who, according to the villagers, is a supernatural "Demon"... and according to Marton's theory, the first serial killer in the history of Kingdom of Hungary.
