Copywriter Lucy Martin lives in Manhattan with her husband Adrian. After two years of trying to conceive, they decide to consult Dr. John Hindle, a prominent fertility specialist and Adrian's former professor. Following a successful insemination procedure developed by Hindle, Lucy discovers during an ultrasound that she is expecting triplets: two boys and a girl. However, Hindle recommends a selective reduction, suggesting that either the twins or the daughter must be terminated to ensure a safer pregnancy. After much deliberation, Lucy and Adrian opt to keep the girl.
Lucy then joins a support group for expectant mothers, where she befriends Corgan, who conceived through IVF. She becomes intrigued by Grace Singleton, a spiritual midwife with a natural approach to childbirth, which she discovers online.
During the reduction process, Lucy loses consciousness, only to overhear a conversation between Adrian and Hindle upon regaining awareness. Following this, she experiences excessive bleeding, which Hindle dismisses as a normal concern. The relationship between Lucy and Hindle becomes strained when Adrian presents him with an award, raising Lucy's suspicions about Hindle's true intentions.
While investigating Adrian's office, she uncovers a safe and confides in Corgan about her growing unease, suspecting that Hindle has ulterior motives regarding her unborn daughter, and that Adrian may be complicit. As Lucy faces further complications in her pregnancy, Hindle attributes it to antenatal depression and prescribes medication. However, after a disturbing dream revealing a connection between Hindle and Adrian, Lucy discovers damning evidence in Adrian's safe indicating she is under surveillance. Corgan, worried for Lucy's safety, advises her to stay calm and report her findings to her lawyer husband.
During a baby shower, Lucy is unsettled to learn that Corgan knows her real identity, despite it never being disclosed. Corgan gifts her the first edition of Peter Pan and Wendy, and Lucy perceives the book cover transforming into an ominous bloodstain. Confronting Corgan leads to the revelation that she is also consulting Hindle and had shared Lucy's file with Adrian, who denies ever owning a safe.
As Lucy prepares to give birth, she faces contractions and opts to visit Grace for assistance. Consequently, she realizes she is birthing the male twins and not the daughter. However, before the second twin can be delivered, Grace insists on a hospital visit due to heavy bleeding. Yet, Adrian insists on returning to Hindle's practice, where the second baby is ultimately brought into the world. With feelings of disillusionment and despair mounting, Lucy confides in Grace, only to discern that her imagined image of Grace as a maternal 'goddess' was largely an elaborate fantasy.
Determined to confront Hindle, Lucy ventures to his clinic, where she is horrified to learn that Adrian plans to join Hindle’s gynecological practice. While investigating a restricted area labeled "The Lab," Lucy encounters her discarded placenta and realizes the grim truth about Hindle's manipulations—he used his own sperm for insemination, aiming to propagate his own lineage. In a fit of rage, she fights back against Hindle, even after being attacked by a nurse named Dawn. Lucy manages to overpower them both, demolishing Hindle's sperm vials in the process, and makes a definitive choice by taking the female fetus with her.
Upon returning home, she envisions herself releasing the twins to float out the window—a nod to the recurring motifs of Peter Pan. When Adrian arrives and makes justifications regarding his collaboration with Hindle, Lucy, unsatisfied and borderline hysterical, demands he leave with the twins. In a surreal moment of vulnerability, she tries to breastfeed the female fetus, encountering a vivid hallucination that it begins to suckle, encapsulating her inner turmoil and desperation.
