Brian de Palma knew his 1970s audience. When choosing what film to see at the cinema (if you had a choice in those days), it was difficult for many men to persuade their girlfriends to choose an out-and-out erotic movie. (The cinema scene in 'Carry On Camping' gives you some idea of the prevailing attitudes.) So, much like the Hammer movies, de Palma wrapped up the sex in a glossy horror thriller coating. Bizarrely, girls found it much easier to tell their mums that they'd be going to see 'The Texas Chainsaw Massacre' than 'Swedish Nurses Get Hot', or whatever.
But watching this movie with 25 years of hindsight, when people tend to be more open about sex, you have to wonder what was the point of this film, and what was an actor as good as Michael Caine doing in it. Angie Dickinson, another highly paid actress of the era, is also in it, but frankly her death is so badly acted that you could fairly say she deserved this film.
De Palma is a great user of that "Actually it was all a dream" device that we're warned to avoid in creative writing classes. So we get two dream sequences -- each with a central shower scene -- which are both flimsy excuses to get the clothes off his leading ladies (Dickinson and Nancy Allen). Despite the partial use of a body-double for Dickinson, these are attractive, gripping scenes, and probably the highlights of the movie.
The less said about the geekish son and the police detective, the better. Allen's redemption from NY hooker to sleep-alone companion (in chintz night attire!) to the son is also less than convincing.