The following version of the book was used to create this study guide: Lourey, Jess. The Quarry Girls. Thomas and Mercer, Seattle, WA, 2022. AZW file.

Heather Cash feels that moving from childhood to adulthood is like being thrown from an airplane with few instructions and that danger lurking everywhere. Heather lives in Pantown, a neighborhood in Saint Cloud, Minnesota. The neighborhood was originally built to house employees of a car manufacturing plant. A key element of the neighborhood is the tunnel system that connects all the homes to each other and to the plant. The factory closed after a short time, leaving all the homes still connected by tunnels. As young kids, Heather and her friends played in the tunnels. The adults sometimes used the tunnels as a means of getting around in bad weather.

As a young teen, Heather is part of a three-member band known as "the Girls." She plays drums alongside her friends, Brenda and Maureen. Brenda tends to be the nurturer of the group while Maureen is the protector. They are both protective of Heather who is a year younger and very self-conscious about her ear that was burned off when she was a child. Heather is also close friends with a boy named Claude. Heather's 12-year-old sister is often at Heather's side.

Heather's home life is difficult. Her mother Connie seldom leaves her room because of an undefined mental health issue that began when Junie was a baby. It was during one of Connie's episodes that she sprayed lighter fluid on Heather's head and set her on fire, resulting in the burned ear. Heather tends to do most of the cooking and cleaning for the family, and she has started to imagine herself as the woman of the house. She has complete trust in her father Gary.

One day, Heather, Brenda, Claude, and Junie are playing in the tunnels when Junie – planning to play a prank – opens the door to another home. Heather and Brenda are shocked to see Maureen performing oral sex on some men. Only later does Heather learn that Gary and his colleague Sheriff Nillson were among those men. Maureen turns up dead, and Brenda's body is discovered a few days later.

Heather knows that police are looking for a man named Theodore Godo who probably killed a woman in nearby Saint Paul. Police believe he might have also kidnapped a young woman named Beth. Heather becomes convinced that Theodore Godo is the man she knows as Ed. Ed has captured the attention of two Pantown boys – Anton and Ricky. When Heather discovers Junie is missing, she correctly assumes that Junie has been lured to a cabin near the rock quarries. She confronts Anton and Ricky and learns they have become killers by following Ed. She has no idea of how to get Junie safely out of this situation. Beth is being held in the basement of that cabin, but she manages to kill Ed with a spike and emerges through a trapdoor just as Heather fears she will not be able to save Junie. Heather, Junie, and Beth run away. Ricky dies in the process, and police arrest Anton.

Hours later, Heather realizes that Sheriff Nillson is going to manipulate the narrative to avoid her role in Ricky's death. She has come to understand that her neighborhood has become unsafe – especially for young girls – because people are willing to ignore the truth. She decides to tell an agent the entire truth, even though it means revealing Gary's role in Sheriff's Nillson's abuse of young girls. As the novel comes to an end, some of those most affected by the events are beginning to heal. Heather and Claude seal up their basement doors, eliminating access to the tunnels. Though Heather briefly worries that this is like turning her back on her childhood and an essential part of her neighborhood, Claude helps her realize they are making the choice to turn away from the darkness and live only above ground.