The novel is written in alternating chapters from the past and present. The novel begins in the present day, with teenaged Mattie Cokey working at The Glenmore, a hotel on Big Moose Lake, to earn money during the summer. One of the hotel guests, Grace Brown, asks Mattie to burn a pack of letters. Later that day, Grace is found dead in the lake near the hotel. Mattie remembers the letters, which she did not have time to burn. She is drawn in by the mystery of what they might say, and she begins to read them. They reveal some shocking information about Grace's lover, Chester Gillette, who checked into the hotel as Carl Grahm. Grace was pregnant with Chester's child at the time, so he killed her.
In the past, Mattie Cokey remembers her life on her family's farm in the Adirondack Mountains of northern New York. In 1906, when she was 16, Mattie dreamed of going to Barnard College. Her best friend, Weaver Smith, shows Mattie's writings to their teacher, Miss Wilcox, who sends an application for Mattie to Barnard. Mattie gets a "full scholarship" but cannot afford to buy the books and a train ticket, or to leave her family's farm. In an attempt to raise the money, Mattie cleans her rich Aunt Josie's house every week, but Aunt Josie refuses to pay her and tells Mattie she is selfish to leave the farm and the family.
Meanwhile, Mattie fends off the romantic advances of her neighbor Royal Loomis. Royal tries to connect with her by giving her a book for her seventeenth birthday. Unfortunately, he gives her a cookbook, which, to Mattie, shows he wants her to be just like other girls.
Mattie makes the incredibly difficult choice to leave the North Woods and go to school in New York City. She leaves in the morning, and the only person she tells is Weaver. She writes three letters, one to her father, one to Royal, and one to Weaver's mother. She has made her peace with Grace because she decided to show the letters to the world so now every one can see the true, tragic story of Grace Brown. She is now ready to leave it behind, and keep her life in the North Woods as a memory.
