Atonement is great, but it's great like a magic trick in that I'm afraid talking about it, even a little, will disturb your experience. I'd like you to have the same journey I did - not knowing much about it, then perhaps giving it grudging respect midway through before finally succumbing fully to its charms as the final credits roll. This is the sort of film that's constructed to impress an audience without expectations of what it is... or what it's supposed to be.
The story, based on Ian McEwan's novel, takes place in the era of World War II Britain. You know what? That's basically all I'm going to tell you. You've got the poster to work with, you know "it" girl Keira Knightley is in the cast, and you know it's an Oscar contender. Giving you any more detail about the specifics of the plot would just be cruel. I'm not a fan of the plot point recap in reviews anyhow. You deserve better than that.
If I had to compare this to something I would say Pride and Prejudice could be called a distant cousin. This makes sense stylistically as Joe Wright directed both. I'd say Keira is better in Pride but the story is better in Atonement. Well, maybe not better, but certainly more complex, and with more depth. Pride and Prejudice was one of my favorite films of 2005 and this is one of my favorites of 2007 - so comparing them is a bit like putting two teacher's pets side by side. Both deserve an audience.
If you're looking for reasons not to see this I can surely provide them. There aren't catchphrases. There isn't a scene where our protagonist runs from an exploding building. The story isn't simply A-to-B. You have to factor in character motivations and make your own peace with everything that's going on. I'm sure the studio wouldn't want this sort of critique out there but this is a thinking person's film. It has as much going on in the things not said as it does on with what the story chooses to reveal. It's an elegant effort to say the least.
Atonement is just a really beautiful film, and I hope I haven't raised your expectations by saying so. Everyone should get to experience this one fresh, for the moments where film comes off as sweet as this are few and far between.