If There Be Thorns, a sequel to
Flowers in the Attic, is told from the perspective of two half-brothers, Jory and Bart. Jory, fourteen, is sensitive, artistic, and good-natured, while Bart, nine years old, is odd and prone to jealousy and fits of rage. Both boys live with their mother, Cathy, and her husband, Chris. Cathy has been married twice before, to Jory’s father, Julian, and Bart’s father and Chris’ brother, Paul. Both previous husbands died some time before.
Bart and Jory play near an old, abandoned mansion next door. One day, movers arrive at the mansion, and the boys see that the new resident is an old woman dressed all in black. When Bart is alone, the old woman invites him into her home, asking him to call her “Grandmother” and plying him with treats. Jory goes looking for his brother and encounters the old woman, but distrusts her. Bart and the old woman quickly become close, though she urges him not to tell Cathy of their friendship. Grandmother’s butler, John Amos, also takes an interest in Bart, confirming that Bart and the old woman are related and giving Bart a diary that once belonged to Malcolm, Bart’s great-grandfather.
Meanwhile, family troubles are mounting. Cathy, a dance instructor, wants to adopt the infant daughter of one of her students, who is close to death. Chris is opposed to the idea, saying the child should be adopted into more stable circumstances. The baby, Cindy, comes to live with the family through an informal adoption. Bart, once the youngest, is enraged. He asks Grandmother for a pony, but she buys him a puppy instead. Bart grows anxious about the puppy’s loyalties, worrying that it will prefer someone else to him. Jory discovers Bart and the puppy, now named Apple, and decides not tell their parents, reasoning that Bart is lonely. Jory is concerned about all the time Bart is spending at the mansion and goes to confront Grandmother, whose name is revealed to be Corrine. Corrine explains that she once had four children, born of a forbidden relationship. Her love of another man led to her to do terrible things to her children. Jory sees a just-arrived oil painting of Corinne in her younger days, and notes that she closely resembles his own mother. Corinne reveals that her second husband was named Bart, and that her daughter seduced him.
Jory’s dog, Clover, goes missing, and Jory suspects Bart of killing him. Jory also overhears strange conversations between his parents and knows that they are keeping terrible secrets. Bart has an accident in the garden and is rushed to the hospital because of blood poisoning, but not before finding Clover’s corpse in the hollow of a tree. While Bart is in the hospital, Jory goes to check on Apple and finds the dog starving. After he recovers, Bart goes to John Amos, who declares that Bart is just like his great-grandfather. He makes Bart swear to take revenge on those who did Malcolm wrong. Bart becomes influenced by Malcolm’s diary, taking note of Malcolm’s simultaneous hatred of and lust for women, his feelings of superiority, and his indifference to others’ feelings.
Mirroring an incident described in Malcolm’s diary in which a young Malcolm attacked and disfigured a female classmate, Bart cuts all of Cindy’s hair off with a knife. Cathy, meanwhile, is preparing to perform a dance piece she’s choreographed. During the performance, she falls, undergoes emergency surgery, and is told she will never dance again. Chris encourages her to pursue other passions, such as the book she has been writing about her childhood. Jory feels neglected after Cathy’s sudden turn to author, and Bart is angry. Cathy suggests locking Bart in the attic to improve his attitude. Jory tells Chris about Bart’s friendship with the woman next door, and Chris goes to confront her. He discovers that the woman is his and Cathy’s mother. Chris and Cathy are revealed to be siblings who developed an incestuous relationship after being locked in an attic for years by Corrine. Corrine places the blame for Bart’s increasingly erratic behavior on John Amos, who is blackmailing her. Jory discovers Apple’s starved corpse.
Bart continues to unravel. He overhears Corrine offer to leave the mansion if Cathy and Chris will let her take Bart, and Bart oscillates between remorse and Malcom-esque threats when talking to Cathy. Bart steals Cathy’s incomplete manuscript, reading the pages and sharing them with John Amos. Chris worries that Cathy is becoming lost in her own trauma, but she refuses to stop writing. From Malcolm’s journal, Bart learns that his family’s history of inbreeding is long—Corinne married her half-brother, fathered by Malcolm and his stepmother.
Jory’s paternal grandmother, a dance mistress named Madame M., comes to visit. She asks Jory about Bart’s “father,” Paul, wondering why he isn’t ever around. Jory realizes that his mother has lied, pretending that Paul is still alive and concealing her sexual relationship with Chris, whom Madame M. knows to be Cathy’s brother. Madame M. discovers this and confronts Cathy. Bart witnesses Cathy and Chris having sex and begins to doubt whether he is Bart of Malcolm. Bart reveals to Cathy that he knows her secrets, thanks to the people in the mansion. Cathy goes to the mansion, recognizing her mother before both women are assaulted and locked in the cellar by John Amos. Bart promises not to reveal where they are. Chris and Jory become desperate to find Cathy, but Bart stays silent. Bart spies on Cathy and Corrine, who reveals that John Amos killed the animals and warped Bart’s mind. A candle Bart brought falls into the cellar and sets fire to the room. Bart tells Jory and Chris where the women are. Jory fights against John Amos and rescues Cathy as the mansion burns, with John Amos and Corrine inside.
Later, Jory reveals that despite getting a new puppy and pony, Bart is still flying into rages and wishing to rule with an iron fist as Malcolm once did. Bart notes that Chris and Cathy are no longer sexually engaged, but only hold each other at night. Cathy has finished her book, which is being published soon. She sees Bart intently reading a red journal he claims is Malcom’s, though that journal burned in the fire. This new journal is completely blank.