Although made rather more recently, the film harks back to 1968 Paris. Then post-de Gaulle, pre-EEC expansion and pre-Euro and facing political and student unrest, France was then very uncertain where it stood on the world political stage. It was not yet 'in amongst the big boys' which is where it sees itself today.
The male subject of the movie Matthew, an American, is in Paris on a one-year course to study the language. During the early days, he by chance encounters a student protest where he encounters an attractive young woman who has seemingly chained herself to some railings. When the protest develops such that the police retaliate against the protesters, he gets caught up in the process. The girl's brother arrives and they return to the siblings' home where their parents are about to depart on holiday. The siblings invite their guest to stay overnight when he later by chance encounters the pair sleeping side-by-side, nude or semi-nude, after their Anglo-French parents have departed (the mother is played by Anna Chancellor, 'Duckface' in Four Weddings and a Funeral).
The siblings have a major interest in classic cinema, especially American movies, and one of their games is to test the other's knowledge. Matthew shares their interest in film and he becomes part of that game which can involve a forfeit, sometimes sexual, should an answer not be known or incorrect. Needless to say, a three-way sexually-motivated relationship later develops as the sister (played by Eva Green) induces and seduces the young man into their games, all of which had been completely private between the pair and very much unknown to their parents. Theo and Isabelle have a history of mutual curiosity and there is the inference of sibling incest, which is later seen to be false as it is Matthew who is Isabelle's first true lover. It appears that Isabelle had never been on a date and possibly the sibling's mutual curiosities are driven by a lack of external relationships with the opposite sex.
There is a scene, roughly mid-way in the movie, where Isabelle, apparently in deliberation, grossly overcooks and burns a meal. Finding it quite inedible, and devoid of funds having cashed all the cheques their parents had left for their upkeep in the weeks ahead, Theo later rummages through what is presumably a restaurant's rubbish pile of discarded food waste to bring home a mixture of partly edible and inedible garbage. Both activities seem to be another side to their gameplay.
The movie attempts to show the changes in personality and attitudes that develop as the relationship expands.
The largely French cast, the major players at least are all very fluent English speaker, although there are moments where they will speak French amongst themselves. Those parts are all sub-titled.
At times all very innocent and mundane and explosive at others, the movie is rather more explicit and thought-provoking than is usual, even for a French movie. I had the impression that it was a challenge for its young stars as is it sexually-charged almost from beginning to end.
Although I am of greater age than those portrayed here, I believe from comments made by younger relatives and by those of friends, that many of today's university students may well see elements of their own lives here. It may perhaps cause concern in some parents to know what their little darlings may be doing behind their backs.
Not quite a French movie, or an American one, but somewhere in between. It should perhaps fail but it does not!