In the summary, I'm only quoting IMDb's storyline for this film. This is all you need to know about its plot.
I just like Ozon's films. I have my favourites, but in all of them I find what I found on Une nouvelle amie: Good story, provocative, even thrilling; good script; well filmed; very nice cinematography; excellent performances. Both Romain Duris and Anaïs Demoustier fit perfectly in their roles. I'd already seen Duris in a couple of films where he provided very good and convincing performances (I now recall De battre mon coeur s'est arrêté; very nice), thus I was happy to find him again (I watched the film only knowing it was by Ozon; that's all I needed to know) and, at the same time, quite surprised about his role. I knew nothing about Anaïs Demoustier and just loved her. Not only she gave a very nice performance but she is also beautiful.
If you are a Ozon fan, like myself, you cannot miss this film.
If you just like plain good cinema, give this film a try. You will enjoy it.