During a birthday party attended by relatives, Cameron Smith and her mother Patricia are visited by their aunt Mia and her daughter Angela. Patricia is wary of Mia because the latter was arrested after she had killed someone in self-defense and placed his body in a closet.
Patricia suffers an aneurysm, and Cameron ends up in Mia's custody. Mia brings her to her home where she lives with Angela and her husband Chris. Things take a dark turn when Mia has Cameron placed in the basement with several other captives: Nancy, Joanne, and Harland. After Chris gives them drugged Kool-Aid, Mia sells Cameron, Nancy, and Joanne out for sex. After recovering from her aneurysm, Patricia tries to get Cameron back with help from social workers. They end up finding out about Aunt Mia's criminal record, but when they track down Mia's house, a neighbor tells them that they have left town.
In Nashville, Tennessee some years later, Mia, Chris, and Angela have settled down with Cameron, Nancy, Joanne, and Harland placed in the basement. As a result of Nancy and Joanne giving birth from the sex work they are put in, Aunt Mia sells their babies on the black market to make ends meet. While at a bar, Mia entices a medical school graduate named Diana into coming to her house, and imprisons her in the basement to help with the deliveries. Joanne goes into labor, but the baby ends up stillborn. Diana tries to escape, only to slip and fatally hit her head on the steps. As Chris is instructed to bury her body, Mia keeps Cameron in line by promising to frame her for Diana's death unless she remains obedient.
While shopping for groceries, Mia meets a homeless man named Pastor, a war veteran who she hires as a pimp. Pastor takes Cameron, Nancy, and Joanne to their assigned clients every night. Nancy's client is a police officer, who is told about Mia but decides to do nothing. After Joanne gives birth, Mia confiscates the baby. While Angela goes back to Atlanta after an argument with her mother, Cameron persuades Chris into letting her care for the baby while Mia naps. Behind his back, Cameron takes the baby down to see Joanne.
One day, the police officer client brings Nancy to Mia, claiming that she tried to escape, causing Mia to have Nancy locked in the attic. Cameron regularly visits her to bring her crackers and water after secretly drugging Pastor, but Nancy advises Cameron not to visit her again. One day later, Cameron finds Nancy dead from heatstroke. When Mia, Chris, and Pastor find Nancy dead, Pastor states that he did not sign up for this. Mia reminds him of their agreement and takes everyone back to Atlanta.
At Angela's latest house, the basement proves too small for all the captives, so Angela locks Cameron in the closet with Mia claiming that she is doing this to make her family strong. Chris states to Mia that he can't find another house with a big enough basement and gives some water and crackers to a locked up Cameron.
Sometime later, Angela's landlord has become nosy following some smell complaints from the neighbors. After a visit at the house, he hears a sound from the basement, breaks open the door and finds Harland and Joanne, whereupon he calls the police. Later that night, Mia, Chris, Angela, and Pastor are arrested. Cameron is freed and is finally reunited with her mother.
Five years later, a remorseful Mia is up for parole with Cameron and Patricia in attendance. When a parole board member asks if anyone would like to say anything before they pass their verdict, Cameron takes the chance to tell Mia that what she said to her before locking in the closet is not making her family stronger and calls her evil. As Mia is led back into prison, a postscript states that the perpetrators that inspired this film received lengthy prison sentences and the city reached a settlement with those that they had failed to help.
