This is where enotes really comes in handy. First of all, do not just watch the movie! Your teacher will know instantly that you didn not read the book. Second of all, don't just read the summary instead of reading the book. You need to actually read both. The summary will help focus your reading.
Here you go:
A brief summary of Atonement is that two girls are on different ends of the same experience and, as a result, the wrong man is married to one of the girls and the wrong man is accused of being a vicious perpetrator by the other girl. World war intervenes, and there is the possibility of atonement. The ending reveals whether atonement was a reality or a insubstantial wisp of air that will soon be lost with all memories as the wrong girl--now a mature woman--suffers a degenerative illness.
I hope you plan to use the study guide as a helpful tool as you read the novel. I just finished reading it and enjoyed it very much, both the storyline and the writing style. Here at eNotes we offer many tools for both students and teachers, and I'm including a link to a page I think you'll find very helpful. From it you can read a general summary as well as more detailed chapter summaries. Happy reading!