Her conniving ways begin upon seeing her sister in a fountain, soaked to the skin in nothing but her undergarments. Supposedly only seen by the estate's gardener Robbie Turner.
Later, Robbie writes two letters to Cecilia
"You'd be forgiven for thinking me mad - wandering into your house barefoot, or snapping your antique vase. The truth is, I feel rather light-headed and foolish in your presence, Cee, and I don't think I can blame the heat. Will you forgive me? Robbie
"In my dreams, I kiss your cunt, your sweet wet cunt. In my thoughts I make love to you all day long."
Unfortunately, the second is mistakenly hand-delivered by Briony, who also reads it before the intended, causing disturbing fantasies which are vindicated after catching them in a compromising position. To complete this hellish night she points the finger at Robbie, wrongly accusing him of her cousin's rape.
While this one summer day defines her forever, the only thing that sets her apart from other fatales is that she eventually realizes her wrong doings and spends the rest of life in shame trying to atone for them.