I finally got around to reading this. I saw the movie version a few years ago and loved it. I remembered it perhaps too well, so wondered if I would enjoy the book, knowing the ending.
It really is very different. The book is long - 480 pages in my paperback version - so covers a great deal more ground.
As in the film, the book essentially starts out with 13-year-old Briony Tallis viewing, by accident, an incident involving her older sister Cecilia, and Robbie Turner, the son of the family's housekeeper. It is 1935. Cecilia's father favored Robbie and was happy to pay his tuition to top schools. Cecilia is home from college for a break and has uncertain feelings about Robbie.
Briony misinterprets events. Not just the triggering incident but later events. As a result, she tells tales about Robbie, and her actions reverse Robbie's promising future. As we follow members of the family, and Robbie, into later years, we find that both Cecilia and Briony have gone into nursing, but are separated, have not seen each other in years. Robbie has gone into the military and is wounded. As a young student nurse, Briony steals time to write, her love from years ago. She later regrets not writing down the details of her days in nursing, but instead inventing stories that glide over the details. When we visit her again in old age, she is a famous writer and is celebrating her birthday with remaining family members, and she thinks back on these days.
Did she ever atone for her bad judgment as a thirteen-year-old? I leave it for you to find out.