Facing financial destitution, Corrine decides to move herself and her children back to Foxworth Hall, her family home in Charlottesville, Virginia. Corrine begins to write letters to her mother, Olivia, persuading her to let her and the children stay in the giant mansion. Corrine tells the children that her parents are very rich, but were upset with Corrine for an unexplained reason and so she had not seen them in years. Olivia agrees to Corrine's wishes as long as the children are hidden; she does not want their grandfather, Malcolm, to know about them. Corrine and her children move out of Gladstone without a goodbye to any friends and take a train to her parents' mansion. They are dropped off by the train in the middle of nowhere and end up walking to Foxworth Hall.
When the children are settled into a room in the attic, Corrine goes off with her mother and promises to return the next day after she has spoken with her father. She believes seeing him in person will win him over. When she returns to her children, she has been savagely horse whipped (willow switch) by Olivia, her mother, who explains to the children that their parents were half-uncle and niece; their father had been Malcolm's half-brother. If Corrine has any hope of gaining her father's approval, the existence of the children must be kept secret. The children are told that they must remain in seclusion in the end bedroom and the attic of their grandparents' vast mansion until Malcolm’s death, and by then, Corrine will get her father's inheritance and provide for the children's dreams.
At first, Corrine lavishes the children with expensive gifts and promises of a bright future. However, as time goes by, she slowly loses interest in her children, particularly Cory and Carrie, who have almost stopped growing due to the stress of being locked inside. The children are physically and emotionally abused by their grandmother who calls them the "devil's spwan" and threatens them with severe punishment if they disobey her rules. Corrine continues to favor Chris, though this love for her eldest does not motivate her to free her children. After the first year, she stops seeing her children for months, leading Cathy and Chris to think something has happened to her, but Cathy later suspects her mother has abandoned them.
The children initially spend most of their time decorating the attic to make it less scary for the twins. They turn it into their own paper-made garden with flowers. Chris fashions a swing for them, to make the flowers move as if there is really a breeze flowing through the attic. As years pass, Cathy practices ballet and Chris realizes his dreams of becoming a doctor. They also become active readers in every book they find in the attic. Corrine's abandonment forces the children to rely on one another for comfort and friendship. This leads to the formation of a new family unit, with Chris and Cathy assuming the roles of mother and father for their beloved twins. Chris and Cathy resolve to teach the twins schoolwork in a makeshift school room in the attic.
After nearly two years of confinement, Cathy and Chris begin to enter puberty. Cathy becomes curious of the changes in her body; in one incident, she is admiring her naked self and Chris accidently walks in on her, and after getting over the initial shock, he tells her how beautiful she is becoming. The grandmother catches Chris watching Cathy and gives them an ultimatium: Chris must cut off Cathy's hair or all four children will starve two weeks. When they refuse to comply, she drugs Cathy in her sleep and puts tar into her hair, which forces Chris to cut it off. The resulting starvation forces the children into desperate measures; Chris cuts his wrist and offers his blood to feed to the twins and guts mice for him and Cathy to eat. Before they can eat the mice, the grandmother leaves them a basket of food, with additional powdered doughnuts.
Months later, Corrine suddenly returns and reveals that she has married her father's attorney, Bart Winslow, and was away on her honeymoon. Cathy and Chris are angry that their mother was away and didn't care that they nearly starved, but she shouts at them for being selfish when she provides necessarities for them. The grandmother continues to abuse them, and even whips both Cathy and Chris when he talks back at her. Due to their confinement, Cathy and Chris become sexually attracted to each other. They also begin plotting an escape. They make a shape of the attic's key in soap and carve a wooden copy. To finance their escape, they secretly steal money and jewelry from their mother and stepfather. One night, Chris is ill, so Cathy goes alone. She encounters her sleeping stepfather. Curious and confused, she kisses him. Chris finds out about the kiss when he overhears his stepfather telling his mother about what he thought was a dream. Chris rapes Cathy in a jealous rage, but later feels guilty for it and begs for forgiveness. Cathy forgives him because she knew he didn't mean to do it and says that she wanted it too. Then, Chris professes his love to Cathy, and although she reciprocates his feelings, she is unsure how to respond.
Soon after, Cory becomes seriously ill, and after Cathy angrly persuades Corrine to take him to the hospital, he later dies, leaving the older children devastated. Now desperate, Chris plans to take whatever money he can find in his mother's suite, but discovers that Corrine and Bart have left Foxworth Hall for good. Chris tells Cathy that he found out that their grandfather has been dead for nine months after eavesdropping on the butler, John Amos. Chris also tells her that he heard their grandmother has been leaving food with arsenic to kill the mice in the attic. Realizing they are the "mice", Chris and Cathy take Carrie and slip out of Foxworth Hall before dawn to catch the train to Sarasota, Florida. At the train station, Chris reveals the final horror: their grandfather's will said that their mother would be disinherited if she borne children from her first marriage or has any in the future. Their grandmother started leaving the food for them nine months ago when their grandfather died and the will was read, therefore, it was their mother who made the decision to poison them.
They abruptly decide not to go to the police, at the risk of being separated. Their priority is to be there for Carrie and survive on their own. Cathy is very angry and desperately wants to take revenge on her mother and grandmother, but decides that at the moment, the priority is to get someplace safe and to get help for Carrie. She does declare that one day, she will get her vengeance. At the time of their escape, Chris is nearly 18-years-old, Cathy is 15-years-old, and Carrie is 8-years-old.