Very much a Lolita story, only with a twist... sort of.
This is the story of Giulio (54), and Francesca (17). The two have an... affair, I guess you would call it. Giulio is married, and has a daughter the same age as Francesca. None of this really matters, and the movie would play out the same way with his family life cut out. In fact, several parts of the movie could have been cut and it would have changed nothing. That's really the problem: there's no real story here aside from old guy with young girl. The end.
I mentioned a twist. Turns out Giulio might be Francesca's father. It's never determined. Not really a spoiler, since nothing comes of it. One would think there would be a resolution or at least a point to that being a large part of the plot. But, no, it's just a detail.
Being that this was/is such a controversial film, I did a bit of research, and repeatedly, I saw it described as "sexy" and "erotic." It's really neither. Yes, Kinski is nude for a large part of the third act, but the relationship between the two characters feels entirely superficial.
I know there are several movies similar to this, the ones that spring to mind are _Lolita_ (either version), _The Debut_ (1977), and _The Lover_ (1992). All of those had depth. This one does not.
One positive: Ennio Morricone does the score, and it's great. He would later make the score for _Lolita_ (1997). Turns out if you're making a movie about an older guy robbing the cradle, Ennio has you covered.
Overall, the movie was just boring. Aside from the Morricone score, a funky disco song, and Kinski fully nude, the movie is a waste.
Watch _Lolita_ (1997) instead.