Grey, a sixteen-year-old approaching her seventeenth birthday, arrives in La Cachette, Louisiana, for her annual summer visit with her grandmother, Honey, who runs the Mystic Rose bookstore. La Cachette is a remote, stilt-built town accessible only by boat, situated where the Mississippi River fragments into the bayou before reaching the Gulf of Mexico. The self-proclaimed "Psychic Capital of the World," it has 106 residents, all spiritualists and psychics. Grey has spent every summer here since her mother's death by suicide when Grey was eight.
The first thing Grey sees is a MISSING GIRL poster of Elora, her best friend and "twin flame," who vanished three months earlier in February. A terrifying vision strikes Grey: Elora running through rain and darkness, pursued by someone unseen. Grey has never exhibited psychic ability, and the flash shakes her. Grey and Elora belong to a group the locals call the Summer Children: ten babies born to eight families during the same year, between the spring and fall equinoxes. The group includes Grey and Elora (born the same June day); Hart, Elora's stepbrother and a psychic empath who physically absorbs others' emotions; Evie, a clairaudient who hears spirit voices; twins Sera and Sander, psychic artists from a Creole family; Case, Elora's volatile Cajun boyfriend; Mackey, who senses approaching death; and twins Ember and Orli, murdered at age four, thirteen years earlier. A recluse named Dempsey Fontenot was blamed for the drowning deaths but disappeared before he could be caught.
Hart recounts the night Elora vanished: The group went to Li'l Pass to hunt for
fifolet (ghost lights from Cajun folklore), then played flashlight tag. During the game, Elora could not be found. A violent storm blew in, and hours of searching, including at nearby Keller's Island, turned up nothing. Hart tells Grey he feels nothing but Elora's fear through his empathic gift. Grey confesses she has been experiencing flashes of Elora's perspective. Mackey reveals he received a death warning about Elora that evening, but Elora laughed it off. Hart accuses the absent Case of the crime.
Grey investigates. Sera and Sander show her psychic drawings connecting Elora to a large black trunk from Honey's storage shed; the trunk is missing. Case confronts Grey in the shed, insisting he is innocent and revealing Elora was secretly in love with someone other than him. Wrynn, Case's nine-year-old sister who has a gift for psychic healing, tells Grey the
rougarou, a werewolf of Cajun legend, killed Elora. That night, Grey spots a figure with blazing ice-blue eyes outside her window.
Honey recognizes Grey's emerging gift as clairvoyance, inherited from her great-grandmother. At Li'l Pass, Grey encounters the stranger: Zale, a boy her age with sun-blond hair and those unmistakable ice-blue eyes. He gives Grey Elora's blue pearl ring, explaining they became close friends after he repeatedly pulled her from the Mississippi when she threw herself off the dock in January. On the night she disappeared, Zale saw Elora on the dock; she said goodbye and was waiting for someone but refused to say who. Zale reveals he is a Summer Child born in La Cachette with a twin brother, Aeron, who died in a fire, making the original count twelve.
Grey discovers Case's Saint Sebastian medal, Elora's good luck charm, wedged in the shed floorboards with what appears to be a bloodstain. On Grey's seventeenth birthday, Hart reveals Elora pushed Grey away the previous summer to protect her. Hart and Grey share a desperate kiss born of grief. When Hart and Case erupt into a fight, Grey brandishes the medal to accuse Case. Hart, however, senses that Case is innocent. Case explains that Wrynn found the medal beside Elora's body on the dock that night and hid in the shed in terror. Wrynn adds that the attack happened on the boardwalk, not at Li'l Pass; she found Elora dead, tried and failed to heal her, and watched someone return for the body and take the trunk.
Zale reveals he is the son of Dempsey Fontenot. His mother fled with him after the cabin fire and settled in Florida; he returned after her death, seeking the truth about his father. When Zale gives Grey a silver hummingbird hair clip found at Keller's Island, where Fontenot's cabin stood, she recognizes it as the match to one her mother wore. At the ruins, Grey enters a vision: Her mother stands before the cabin as it erupts in flames. Zale's mother flees with the small boy Zale, but Aeron is trapped inside. Honey confirms that the townspeople found Ember and Orli behind the cabin and, consumed with rage, blamed Fontenot. Grey's mother set the fire not knowing a child was inside, and the guilt was a factor in her death by suicide.
As Hurricane Elizabeth threatens La Cachette, Hart reveals Fontenot's skeleton hidden in a barrel on the dock for thirteen years. He recounts what he witnessed at age five: Fontenot appeared cradling Aeron's burned body and unleashed a supernatural storm; Leo, Elora's father, shot him dead; the town concealed the body. Hart argues Zale killed Elora as revenge. Grey searches Elora's bedroom and finds a gold bracelet with a red heart charm, wrapped in a note reading "Soon" in distinctive handwriting, hidden in a spot only she and Elora knew about.
With evacuations ordered, Grey stays behind with Hart to find Evie, who has gone missing. Hart reveals he once threatened Evie's uncle, Victor, at gunpoint to stop his abuse of Evie and her mother, explaining Evie's fierce devotion to him. Grey opens Hart's birthday card and recognizes the handwriting as matching the note from Elora's room. She realizes Hart and Elora were secretly in love and that Hart was the person Elora planned to run away with. Zale confirms Hart never went to Keller's Island to search that night, contradicting Hart's earlier account.
Case and Wrynn arrive. Wrynn has found Evie hiding at Li'l Pass, and Hart goes to retrieve her. Alone, Grey pieces everything together: Hart's lie, Wrynn's terror of him, and Evie's refusal to reveal what Elora's spirit whispers to her. Hart returns and confesses: He and Elora planned to run away, but he lost his nerve and backed out. They fought on the dock. He left her alive but returned minutes later to find her dead, her skull crushed by an anchor chain. He hid her body in the black trunk and sank it in the gator pond. He then reveals the deepest secret: His own father, not Fontenot, drowned Ember and Orli, feeding on their terror as a sadistic empath, and forced four-year-old Hart to carry the bodies to Fontenot's property. Hart's mother killed his father after Hart told her the truth.
During the hurricane's eye, Hart retrieves the trunk from the flooded gator pond. Evie appears with a pistol, revealing herself as Elora's killer: She overheard the plan to leave and, unable to bear losing the one person who saved her from Victor's abuse, attacked Elora with the chain after Hart walked away. Hart promises to stay with Evie and screams at Grey to run. Two gunshots sound as Grey flees toward Zale, who uses his inherited power over storms to carry her to safety as the hurricane obliterates La Cachette.
Two years later, Grey is nineteen, living in New Orleans with Honey. They run a bookstore called the Grey Rose, and Grey attends Tulane University. La Cachette is permanently submerged. Of the twelve Summer Children, six are alive and six are dead. Standing above the Mississippi, Grey senses a familiar electric charge and turns to find Zale, whose offered hand signals reunion and a future beyond the storm.