Written in 2021, This Poison Heart is a YA fantasy novel by New York Times best-selling author Kalynn Bayron, who is known for her retellings of fairy tales and other classic stories focused on Black protagonists. This Poison Heart is a coming-of-age story that follows the life of Briseis Greene as she learns about her magical heritage and comes to terms with her innate talent for controlling plants. The novel focuses on secret backgrounds, hidden talents, and the true definition of family.

Bayron is widely known for her 2020 debut novel, Cinderella Is Dead. This Poison Heart was named to the IndieNext Kids List for May/June 2021, Autostraddle Best Queer Books of 2021 list, and numerous “Best of 2021” teen and YA lists, including recognition from the Boston Public Library, the New York Public Library, Waterstones, and Booklist Editor’s Choice. Bayron has also been honored with the 2022 Randall Kenan Award for Black LGBTQ Fiction.

This guide refers to the 2021 electronic edition published by Bloomsbury YA. Variations in pagination may occur.

Content Warning: Both the source text and this guide contain descriptions of violence, racism, and the death of a parent by violence.

Plot Summary

Briseis Greene, who has the secret magical ability to control plants, is working in her adoptive mothers’ flower shop one day when a customer discovers that the shop is out of stock and cannot fill his regular order of white roses. Taking a risk, Briseis quietly uses her magic to grow some perfect blooms even though she often finds it difficult to control and hide her powers. Despite her mom Thandie’s order to relax and enjoy her summer, Briseis plans to continue working at the flower shop, which is struggling financially. She also has a secret project: growing the highly poisonous water hemlock. However, as she is dissecting the hemlock, she is startled by her other mom, Angie, and accidentally cuts herself, inadvertently infusing the wound with poison. Though the poison should have killed her almost instantly, Briseis suffers no ill effects aside from a cold sensation.

Several days later, Briseis and her moms hear from an estate attorney named Mrs. Redmond that Briseis has inherited a house from her biological aunt. Located in an upstate New York town called Rhinebeck, the house is fully paid for and represents an unexpected financial windfall for the struggling family. However, Briseis worries about the prospect of controlling her talent if she is in such close proximity to nature. Even so, the Greenes decide to visit the house for the summer. They find it grand if unkempt, and the interior includes many old belongings from a family of Black women who resemble Briseis. Briseis reads notes that her deceased aunt, Circe, left to her. (Later in the story, however, she will learn that these notes were forged by Mrs. Redmond, who is not a real attorney but is instead a member of a competing magical family who wants to steal a poisonous plant that Briseis’s ancestors have guarded for millennia.) While Briseis explores a safe that contains another letter, she sees a stranger standing outside, but when she and her parents go to investigate, nobody is there. They call Public Safety Officer Khadijah Grant, who promises to keep a lookout.

Further explorations of the house lead the Greenes to a book of poisonous plants and a map. Following the map, Briseis finds a hidden garden behind a path of stinging plants; she can only access the area thanks to her powers. While searching for the garden, she encounters a man carrying a machete who calls her “Selene,” the name of her biological mother. When Briseis grows frightened, the plants respond to her fear and attack the man. Dr. Grant arrives and takes the man (whom Briseis later learns is named Alec Morris) to the hospital.

The next day, the Greenes go into town, where Briseis meets Karter, Mrs. Redmond’s son. The two teens begin a tentative friendship. Meanwhile, Briseis continues to explore her new home and the secret garden, which contains a second secret garden behind a gate. Inside this inner sanctum, the plants are so poisonous that only someone with Briseis’s powers can safely breathe the air. As her friendship with Karter develops, the two visit Alec in the hospital. He apologizes for frightening her and explains that a plant sold in Briseis’s family’s apothecary is the only effective remedy for diabetes-related sores. Briseis promises to obtain some of the plant for him and resolves to reopen the apothecary to help her family earn some additional money. That night, the mysterious stranger appears again, this time at the door; she introduces herself as Marie, Alec’s granddaughter, and Briseis feels attracted to her.

When Karter visits the following day, Briseis decides to tell him about her powers and shows him the garden. Curious about the numerous representations of the mythological figure of Medea in the house, Briseis contacts Angie’s friend, Professor Kent, who is a classics professor. Professor Kent suggests that there is often truth behind ancient myths. In town, Briseis meets another magic practitioner named Mama Lucille and visits Mrs. Redmond, hoping to obtain more keys. While there, they hear a news report announcing the death of a local resident. (In Chapter 28, Briseis will learn that Mrs. Redmond killed this woman.) At home, Briseis discovers a secret room behind her bedroom fireplace, in which she finds an ancient sheet of paper and another key.

Briseis spends the evening at Marie’s house, noting the oddness of the mysterious girl. Even so, Briseis hopes to initiate a romance with Marie. The next day, Briseis enters a small enclosure at the back of the poison garden, which contains the most toxic plant that Briseis has ever encountered, the Absyrtus Heart. In the apothecary, Dr. Grant’s father, Isaac, shows Briseis how to use her powers to “transfigure” poisons. Briseis is unnerved by her growing knowledge of her powers, but she puts her worries aside to attend a movie with Karter.

While at the theater, they are attacked by four men and only narrowly escape. When they report the attack to Dr. Grant, he surmises that the attackers are likely local magic practitioners who want to take advantage of Briseis’s magic. Later, when Briseis and Marie visit the graves of Briseis’s biological mother and aunt, the same four men attack again. This time, Marie and her bodyguard, Nyx, kill them all, and Marie admits to having gained immortality when Briseis’s ancestor used the Absyrtus Heart to make the Living Elixir and heal her of the bubonic plague. Briseis struggles with these many revelations but resolves to visit a local coroner to get more answers about the deaths of her biological family, as they have not been accurately reported.

Lou, the coroner, tells Briseis that many of her ancestors have been killed by violence, and their murders remain unsolved. An email from Professor Kent identifies the parchment from Briseis’s secret room as an ancient and long-lost document that contains the first known reference to Medea. Briseis takes the paper to Alec, who knows how to translate the ancient text; he offers an adaptation on the myth of Jason and Medea, wherein Jason killed Medea’s brother Absyrtus and cut his body into six pieces. These pieces grew to become a toxic plant that can only be tended by Medea’s family. The plant must be fed with blood and moonlight. Briseis resolves to show the Absyrtus Heart to Marie and Karter because she is tired of keeping these secrets to herself.

In the Absyrtus Heart’s enclosure, Briseis feeds the plant a drop of blood, and it moves. Seeing this, Karter becomes frightened and bolts, and Marie and Briseis are injured in their attempts to save him from the poisonous plants. The next day, after finding an altar to the Greek goddess Hecate in the apothecary, Briseis tells her moms everything, relieved to have their support. Karter returns that afternoon and urges the Greenes to leave Rhinebeck, citing an unspecified danger. Shortly afterward, Mrs. Redmond arrives, looking bruised and battered. (Unbeknownst to the Greenes, she was injured when trying to break into Briseis’s secret garden.) She tells the Greenes that they are being evicted. Confused, Briseis inquires at the bank about the house’s finances and learns that Mrs. Redmond stole the documents denoting the house’s ownership and has been posing as an estate lawyer. Briseis hurries back to the house, urging Angie (who has traveled to Brooklyn for business) to do the same. When Briseis arrives, she finds Mrs. Redmond holding Thandie at knifepoint.

Mrs. Redmond admits that she is descended from the mythological Jason; her family has been a rival to Briseis’s biological family for generations. She wants the Absyrtus Heart so that she can achieve immortality. Threatening to kill Thandie if Briseis refuses to handle the dangerous plant, Mrs. Redmond forces the teen to retrieve the Absyrtus Heart and transfigure it into the Living Elixir that grants immortality. Karter arrives to help Mrs. Redmond, revealing that he has been working against Briseis all along. (As it turns out, he was also complicit in the attack they experienced at the movie theater.) After Briseis creates the Living Elixir, a scuffle ensues. When Briseis tries to grab the vial of elixir, Mrs. Redmond poisons Thandie, killing her. Suddenly, the goddess Hecate arrives with a giant dog, which drags Mrs. Redmond to the underworld while Karter flees.

The remaining specimens of the Absyrtus Heart are hidden in different locations around the world, and Hecate counsels Briseis that if she can gather all six pieces of the Absyrtus Heart within a month, Thandie can be resurrected. Hecate then disappears with Thandie’s body. Overcome with shock and grief, Angie and Briseis try to explain the events of the day to Dr. Grant and Marie. As the novel ends, two women arrive, one of whom is identified as Circe, Briseis’s biological aunt, who was presumed to be dead. The text implies that Circe has been away on an expedition to locate several pieces of the Absyrtus Heart; she carries with her a case from which the sound of a heartbeat emanates, and she tells Briseis that she has many things to teach her.