Judging by the name of the film and by the name of the director, it must be a horror or mystic movie. But, in fact, we have before us a detective story with thriller elements.
The plot of the plot is as follows: at an institute engaged in secret research in the field of genetics, a guard was killed at night, but nothing was stolen. The next day, an employee of the institute dies under the wheels of a train. The investigation begins by Franco Arno, a former journalist and now a blind pensioner who loves solving charades, and connects a young newspaper reporter Carlo Giordani to him.
As a detective, the film is clearly not enough, the study of the personality of the "investigators", there are no "author's" methods of investigation - they are all quite typical.
The positive aspect of the film is a good idea of the motive for the crimes, already somewhat fed up with sexual maniacs or psychos, obsessed with any idea. Still, Argento brightened up this film with his trademark gadgets: he quite realistically removes scenes of suffocation (saliva with blood), the appearance of a dead man in a crypt, which has just begun to decompose, is natural.
Several times during the film, he changed his mind who the killer was. The denouement is unexpected, albeit crumpled.
In general, as a classic detective story, it is a little weak. And Argento falls short of the horror classics, but there is an idea that is usually not in the director's later films.