Six years afterPetals on the Wind, Cathy and Chris are happily married and live in California with Cathy's sons, Jory and Bart. Bart feels lonely and outshone by his brother and parents. One day, a woman dressed in all black moves into the mansion next door and invites Bart and Jory over for tea and cookies, and explains that she is rich but has no family except for her butler, John Amos. She takes a picture of them for keepsake, and asks them to visit her more. Jory declines but Bart agrees after she promises to give him anything he likes. Bart is curious why he has no other relatives and knows that Chris is his stepfather, but Jory says that their maternal grandmother is in a mental hospital and Chris' relatives are dead. Corrine lavishes gifts and treats on Bart, including a pet python, and tells him she is his grandmother, Corrine. John Amos gives Bart a journal belonged to his great-grandfather, Malcolm, and makes him promise to never reveal it to his family. Bart reads how Malcolm referred to beautiful women as sinful and degrading, which envelopes his mind.
One night, Cathy has placed three beds in the attic but denies that she is planning to put the boys in the attic; she is just afraid of losing the boys if her and Chris' secret is exposed. She pleads with Chris to adopt Cindy, whose mother had just died from cancer, and he agrees in order to calm her down about them losing their family. They find Bart missing and discover him unconscious in the woods due to an infected cut on his arm, leading him to be hospitalized. He starts to suspect that his parents are keeping secrets from him, and confronts Corrine over a painting of a boy who resembles Chris because he was told that his mother stopped talking to her before meeting Chris. Corrine confesses that Cathy and Chris are both of her children. Bart starts to withdraw and lash out to his family. After the family dog is found dead, Jory suspects that Bart was responsible (but it is later revealed John Amos was). Bart starts to pressure Corrine to tell him about his biological father. John Amos continues to influence Bart about the sins and hypocrisy of the family. Jory tries to get him to stay away from Corrine because he blames her for the change in him, but Bart threatens to tell their parents that he has been having sex with his girlfriend, Melodie, in the attic. Pressured by Bart, Corrine admits that his real father was her second husband and Cathy seduced him to get back at her and Corrine wants Bart to become the son she never had with his father. Bart has been resentful towards Cindy since Cathy and Chris adopted her, and attempts to drown her but is stopped by Jory. Chris locks him in the attic as punishment and Bart shouts at him as "devil's spawn" and "fornicating his sister", revealing that he knows their secret.
Bart is placed in a mental hospital and says it was Malcolm who tried to drown Cindy, leading the doctor to suspect he suffers from a mild case of schizophrenia. Jory goes to confront Corrine, who tells him that Cathy and Chris are siblings, which horrifies him. Jory then tells Chris that Corrine is living next door, and he goes to confront her. Corrine begs for Chris to forgive her and let her be part of her grandsons' lives, but what she wants most of all is to raise Bart as her own. Chris rejects her and threatens to lock her away forever if she doesn't leave them alone. At Cindy's dance recital, Cathy is distracted when she sees Corrine in the audience and injures her leg when she accidentally falls. Jory's paternal grandmother comes for a visit, but it is revealed that she threatened to turn Cathy and Chris over to the police for incest and take custody of Jory if she didn't give her money for her dance studio. She has decided to take Jory regardless, putting Cathy on edge. She tells Cathy that Jory's current living situation is unhealthy for him and points out how "nuts" Bart is. Bart storms out of the house with Cathy chasing after him and runs to John Amos for help. John Amos tells him that if he can't deal with the sins and hypocrisy then he shouldn't be with Corrine because she is a sinner like his parents due to her committing incest with her half-uncle, which is how Cathy and Chris were born. Cathy goes to Corrine's mansion to find Bart, and finds a portrait of her siblings and herself. She confronts Corrine and orders her to leave her family alone. Corrine pleads for forgiveness but Cathy rejects her and blames her for ruining the life she and Chris worked hard to build. Corrine asks Cathy if she loves Chris, which she confirms and blames Corrine for this. John Amos knocks them both out and forces Bart to lock them in the barn with the plan to burn them alive.
John Amos reveals that he is disgusted with Corrine for her incestuous relationship with her half-uncle and thought she wanted to redeem herself after giving her children up. But now that she wants them back, he will kill them to end the sins in the family. Bart doesn't want to kill them and fights against John Amos and runs off to get Chris. John Amos sets the barn on fire just as Chris and Bart arrive to rescue Cathy and Corrine. Thinking they are about to die, Corrine professes her love to her and her siblings, and Cathy embraces her, confirming her forgiveness. Chris rescues Cathy and Bart tries to save Corrine and is about to be attacked by John Amos. Corrine stabs him with a fire poker and gives Bart enough time to escape, just as she burns to death with John Amos. Realizing how much he loves his parents, Jory decides to remain with his parents and threatens his grandmother that he will never forgive her if she reports Cathy and Chris. Bart calms down, and things start to return to normal. However, Bart is secretly starting to dress like Malcolm and still has his journal, hinting he has not recovered from his madness.