Before continuing, please understand that Dark Chapter is a story about rape, and the damage that such evil violence causes in the psychology of the victim. So please be cautious before proceeding, because the tale is traumatic and dark.
Vivian is a Taiwanese-American on vacation in the UK, near Belfast. One day, a 16-year-old guy brutally rapes her while she is taking a walk. When the authorities arrive to help her, the story details the gory aftermath, the invasive procedures to help her fight against STD's, and immediately, she has to talk about the traumatic incident over and over again, to police and medical professionals.
Her life after that point is grim and surreal. Entire months and years go by, and still, Vivian endures the arduous process of daily life, hoping for some way to make sense of her life. In the novel, the rapist denies his actions and the victim has to endure further trauma when her personal nightmare becomes an issue of public record, and she has to endure the difficulties of criminal prosecution against the boy.
The lawyers try to paint Vivian as the real criminal, and they publicly humiliate her, literally adding insult to injury.
Meanwhile, the author weaves in an alternative story, the story from the attacker's point of view, where she explores Johnny's difficult background and tries to make sense of his actions. It's not clear whether this represents a third-person omniscience, or whether this narrative is imagined by the victim during her countless hours trying to make sense of her violation.