Eighteen women are dead and a horrible serial killer, known as the monster, is on the loose in a small Italian suburb. Criminal psychologist Taccone is convinced that the homicidal pervert is Loris, a small-time thief, who had the misfortune of being seen at an inopportune moment with a chainsaw between his legs. A hapless schnook with a healthy appetite for the opposite sex, Loris is constantly misinterpreted by Taccone, who always comes up with the most deranged analysis for his mischievous behavior. Ultimately, the psychologist baits the trap to catch Loris with Detective Jessica Rosetti, a beautiful police officer who is sent to live in Loris' apartment with instructions to urge him on 24 hours a day. Loris tries to resist temptation, but his surprising restraint only convinces the psychiatrist that not only is he ready to explode, but that Rosetti must do even more....