Briony runs away from her sister and up to her room to get ready for dinner. While there, she tries to write. We learn, without doubt, that Briony opened the letter and discovered the obscenity in its closing sentences (Robbie writes: "In my dreams I kiss your cunt, your sweet wet cunt. In my thoughts I make love to you all day long."). The “c”word was foreign to her, but she correctly assumes its meaning. As she sits and wonders what to do next, Lola enters her room. Briony is aware that the letter along with the fountain incident has been a loss of innocence for her, and she is now ready to become the “adult” writer she has always wanted to be.
Lola begins crying because her brothers are abusing her, mistaking their reason for being stuck in the Tallis house as her fault, and not that of their parents. Briony consoles Lola and they have a bonding moment, moving towards friends instead of enemies. As part to get her mind off her parents and brothers and as part for the need to tell someone, Briony shares the contents of the letter with Lola. Lola identifies Robbie as a “maniac” and suggests they go directly to the police.
Lola heads down to dine after being summoned by Emily, Briony stays to fix her hair and face. Eventually, she makes it downstairs but instead of going directly to the dining room, she enters her father’s study after hearing a scraping noise and a thump come from within.
Briony enters the study and discovers Robbie and Cecilia pinned up in the corner. Her perception is that of an attack and that her sister is being held there against her will. Her sister’s dress is above her knees, Robbie has a hold of her hair and her forearm, and she is being pressed into the corner. Briony calls her sister’s name, who gets out of the compromising position and leaves the study without saying anything to Briony. Briony is left in there alone with Robbie who does not acknowledge her and goes about straitening out his clothes.
After reading the letter, Chapter Ten begins with that exact moment when Briony's innocence is taken from her forever. The opening sentence of the chapter confirms this notion. What is important to note however, is that Briony is still coming across parts of the adult world that she does not fully understand. Just like the scene she witnessed at the fountain from a distance, this time it is a word she reads. She thinks she knows what it means (in fact, she is correct in her assumption), but she is unsure. She interprets the word as "almost onomatopoeic" and this "disgusts her profoundly." The reading of the letter leads to her imagination to conclude that her older sister is being "threatened." Briony's inability to understand the adult world as a child is evident in her act of opening and reading a letter not addressed to her--something a mature adult would not do.
Briony desires to lose herself in the "unfolding of an irresistible idea." This passion for articulation desires onto page is something that she feels herself and Robbie share. However, where Robbie is in tune with the chaos of desire, Briony seeks to "impose order" to it, which will be the tragic flaw in her writing and life (something that is mentioned in the rejection letter from C.C. in Part Three which she must overcome to be a successful writer). Her resolve is a determination to "never forgive Robbie and his disgusting mind." This is ironic for a heroine who spends her own life seeking forgiveness of her own.
Briony's decision to share her information with Lola is what defines her a story-teller. There is a necessity for her to "share a secret" and she does so very easily with the one person she actually hates. Again, it is irony that is presented: Briony shares this secret as a thirteen-year-old with ease and spends the rest of her life struggling with the sharing of another secret, that is the crime she committed.
A book about what is "imagined" and what is "true" pivots on the final scenes of Chapter Ten (the library love-making) and Chapter Thirteen (the rape). Many scenes in the novel are left open to interpretation, and Briony's young and creative mind repeatedly plays tricks on her. She trusts her literary instincts more than her sensory perception. The 'c' word in the letter must mean rape in the library. She forgets that Robbie and Cecilia are real people in the library and instead uses them as a source material for her imagination.