This movie really is just a bad clichée.
Mid aged prison warden (Guillaume Gallienne aged 44) falls for his much younger and very beautiful new female prisoner (Adèle Exarchopoulos aged 23).
The story line is all about his attraction for her. And it's extremely flat and boring. We don't know her objectives. Which could be made in an interesting manner, except it's not. And there is nothing extraordinarily interesting about this story, except that it is set in a prison.
The film is basically a creepy old man lusting after a beautiful young girl. Not exactly new story... There is no depht to it either, it really is as shallow as it sounds.