A sixteen-year-old named Liberty Wells, often called Libby, makes her way back to her family's lakeside home after attending summer camp. Throughout their drive, her mother, Michelle Wells, a romance author, offers Libby the chance to practice her driving skills. Upon their arrival, Libby is introduced to her mother's new partner, John, an aquatic geneticist who gifts her a bracelet adorned with symbols representing a fertility goddess. During their dinner, John elaborates on his work, indicating his focus on helping creatures adapt to evolving habitats.
However, the evening takes a strange turn when Libby stumbles upon some unsettling sounds while wandering into the kitchen, leading her to quickly retreat to her bedroom. There, she finds John lingering at her doorway, causing her to feign sleep. The following morning, the duo engages in an emotionally charged dig where they unearth time capsules buried in the yard, leading to a dispute about Libby's belief that her mother is indifferent towards her grandparents. This heartfelt conversation takes an introspective turn when Michelle reveals her feelings of unworthiness from her father’s side.
Libby's exploration of the lake leads to an unsettling incident where she's bitten by a substantial insect. Fortunately, John is on hand to offer his assistance. Upon returning to the house, she discovers that Michelle is feeling unwell and learns that John has proposed marriage, to which her mother has agreed. Driven by curiosity, Libby seeks John out in the basement, where she becomes privy to his work involving parasites.
As Libby enters puberty, she experiences her first menstruation, which leads to an awkward moment with John when he attempts to clean up the blood with his own shirt. Afterward, while Libby showers, John crosses the boundary of privacy, sneaking into the bathroom to sniff her through the curtain—an invasion that deeply unsettles her. Removing the bracelet John gifted her, Libby notices a troubling red mark on her wrist.
Reaching out to her friend Marley, Libby requests her to visit the house. Later that night, she has a disconcerting vision of John walking into the lake, which is strangely illuminated with red lights. He then reappears dry and explains away his bizarre behavior as sleepwalking. The next day, Michelle sends Libby to town for medication and a pregnancy test. While there, Libby follows a man resembling John with another woman, heightening her concern over his intentions.
Returning to the house, Libby confides in Marley about John's inappropriate behavior, prompting Marley to consider informing Michelle. When Libby eventually catches up with her mother, she learns Michelle is expecting; this news disappoints Libby, who feels ignored as Michelle dismisses her claims regarding John. Upon noticing John's nictitating membrane, Libby tries to warn her mother, but is met with hostility instead.
Not long after, Libby investigates the basement again, only to find the same symbols on her own wrist where the bracelet once was. Further exploration reveals a horrifying sight: Michelle restrained in a tank, connected to an IV, with similar symbols etched onto her wrists and the surrounding walls. Hidden from view, Libby watches as John assists Michelle in giving birth to a moribund creature.
In a flurry of events, Libby contacts the police, only to stumble upon Marley's lifeless body in a water tank, infested with parasites. This revelation sends John chasing after Libby as she attempts to rescue Michelle, but soon she finds herself captured, tied up in the basement. A group of men, all identical to John, begins peeling away the walls to unveil a shocking scene of unconscious women suspended in water tanks. In a grotesque twist, John forces Libby to ingest a bioluminescent substance that he regurgitates.
Libby eventually awakens in a line of water tanks filled with other women, seemingly in a pupae state. As the water levels rise, she starts to panic, banging on the glass, but a smile spreads across her face as she realizes she can breathe underwater. The surreal horrors of her surroundings unfold as she navigates this chilling new reality.
