This film is an extremely entertaining psychological thriller with Carrol Baker excellent as a rich American widow getting away from it all at a secluded Italian villa. Enter smart aleck down on his luck playboy, Lou Castel, who has some golddigging motives behind his slick hipster seduction techniques and has no qualms about how he gets what he wants. He encourages Baker's character's incipient alcoholism and dubious self-esteem, brings in his supposed 'sister' to help drag Baker even further into a maze of sick mind games, drug addiction and group sex, all culminating in a descent into near-madness and self-destructive depravity.
I won't give away anymore, suffice to say that the film delivers on its swinging sixties, Euro sleaze ambience and psychological suspense thriller credentials in spades. Director Umberto Lenzi's work has been, at best, uneven and he's churned out his share of crap. However, he made some quite good giallos and crime pictures in the sixties and seventies and this is one of them (other good films by Lenzi -- SPASMO, SEVEN BLOODSTAINED ORCHIDS, ASSAULT WITH A DEADLY WEAPON, to name only a few).
Unfortunately this film is ofen confused with another okay but not-quite-as-good Lenzi film that was also released briefly here in America under the same PARANOIA title (but is more commonly known, especially on video, as A QUIET PLACE TO KILL).