Over the past year Briony, a fit, bright girl with a passion for stargazing, has become moody and introverted and her friends can't figure out why. They know nothing of Briony's secret life at home, where her apparently respectable stepfather, Colin, is increasingly violent towards her mother, and on occasion, towards her. When Briony tries to help her mother she only seems to make things worse, so she keeps quiet and pretends that nothing is wrong, just like everyone else in the family. But things are getting worse and Briony is scared that one day, Colin will go too far. Briony knows she can't runaway and give up on her mother, but how can she convince her to leave? And what will be the consequence of the scenes she has witnessed for her own budding relationship with lan, who had seemed so different?