The crux of the story is that the younger child sees something she misunderstands and (as a consequence of the misunderstanding) sends Knightley's working class boyfriend to prison. The remainder of the film looks at the aftermath of that incident.
The film reminds me a little bit (at a stretch, if I squint) of what might have happened after the trials in To Kill a Mockingbird or A Passage to India, if they'd be set against the backdrop of the English class system (rather than across racial boundaries).
It was actually an ok film, but I found the particular 'social norms' of 1930s England got in the way of the film. I wanted to yell 'Ok, we get it, now move the story along" on occasions.
Thinking about it, there were a couple of other things that bugged me too. Firstly, the extremely posh and well brought up younger sister (who is 13 remember) comes across an 'expletive' in the course of what she's doing. We are expected to believe she knew the significance of a word I didn't know at 16 and kept the note as evidence for later.
Secondly, James McAvoy (the lower class lover) has an extremely posh accent for someone in his stage of life. I understand he went to Cambridge University, but his accent never changes, even when he's with his friends. I don't know about you but I always find people speak differently at home and because McAvoy didn't I found him less believable.