Romeo a police chief detective is on an obsessive hunt for a serial killer, that he thinks may be Velasco, a professional engineer beyond suspicion. Nobody believes his theory, the man he suspects is too well known, too well to do, very powerful and the inquiry is not authorized. The chief detective is very obstinate and he continues in his inquiry with the help of a young, determined assistant Grazia Negro and his trusted friend Rago, an inspector at the scientific police.