As the book opens, it is several months after her beloved Stepmother's death. Briony believes -because it is what Stepmother helped her to understand- that she is responsible for the many bad things that have happened in her family, including her sister's mental condition and her stepmother's injury and subsequent death. The dreams Briony once had of education and a life of her own are buried under the guilt and self-loathing she feels, as she cares for the sister she feels responsible for. To a certain extent, Briony has withdrawn mentally from the world around her, in order to protect her family from her wickedness. When Eldric -the son of the engineer sent to oversee the draining of the Swamp (in order to advance the railway line)- comes to stay at their house, he befriends her and accepts her for who she is, without any pressure. Little by little Briony's frozen mind begins to accept his friendship.
When the Old Ones' anger erupts at the draining of the Swamp, Briony must strike a deal with the Old Ones to save her sister from becoming their next victim. But saving her sister is going to mean revealing her own wickedness.
There is a somewhat dark, oppressive tone to the story, not only because of Briony's guilt, but because of the Swamp inhabitants, who are not very nice. And yet there are lighter moments in the interplay between Briony and Eldric, and there is the seed of hope, that surely Briony will not be left to wallow in her misery forever, surely she is not as wicked as she thinks she is.