The film begins with Casey Beldon (Odette Yustman) jogging through a park during the winter. While pausing to catch her breath, she spots a glove on the ground. Upon picking it up, she turns to see a pale boy missing his glove. As she looks again, the boy transforms into a dog wearing a peculiar white mask. Curious, she follows the dog into the woods, but it vanishes, leaving only the mask behind. When she attempts to retrieve it, she discovers a clear container buried beneath it, containing a still-developing baby that opens its eyes.
Suddenly, she wakes up on her couch, engaging in a conversation with her best friend Romy (Meagan Good) about the dream. Eager to interpret its meaning, Romy turns to a dream book, explaining that a baby signifies change and renewal, while the dog often serves as a harbinger of death in mythology. Later, as Casey babysits two children, she hears a chilling whisper through the baby monitor urging her to "Look in the mirror, some people are doorways." This eerie message leads her to discover Matty, a four-year-old, repeatedly telling his baby brother to "Keep looking" into a mirror, before striking her and insisting, "Jumby wants to be born now."
As Casey exits the family’s house, she finds the same glove she saw in her dream. After an unsettling night, strange occurrences escalate: she cracks an egg for breakfast, only to be horrified when a giant insect emerges from it. Gazing outside, she spots Matty standing ominously in her driveway. Doubts arise when Casey discusses her unnerving experiences with her boyfriend Mark (Cam Gigandet) and her friends, leading to Romy mentioning an old superstition regarding babies seeing their reflection.
Throughout her day-to-day life, Casey grapples with increasingly unsettling hallucinations and bizarre incidents, including a visit to an eye doctor, who reveals that her heterochromia could stem from past trauma. During a series of reflections on her family history, Casey learns from her father that she had a twin brother who died before birth, named Jumby—a revelation that links back to her unsettling dreams.
Determined to uncover the truth, Casey investigates her family's past, leading her to a nursing home where Sofi Kozma, her grandmother, resides. Sofi’s discussions regarding the link between twins and spirits reveal that Casey's dreams and visions may not just be figments of her imagination, but rather a manifestation of a long-buried trauma tied to her family’s history with a malevolent spirit—a dybbuk that has been haunting her.
Her fears intensify when she discovers the existence of a Book of Mirrors that may hold the key to her liberation from the spirit. Casey enlists the help of Rabbi Sendak (Gary Oldman), who reluctantly agrees to assist her after seeing her distress. Under his guidance and with assistance from a priest, Arthur Wyndham (Idris Elba), they prepare to execute an exorcism in the abandoned mental institution where her mother once stayed, a site steeped in painful memories.
As the exorcism proceeds, chaos ensues, with spirits manifesting and the lines between reality and the supernatural blurring. The terrifying climax pushes Casey to confront the demon haunting her while unraveling her family's painful legacy. With a profound understanding that her mother's struggle with the spirit had been a desperate battle, Casey strives to break free from the cycle threatening to ensnare her.
In a poignant conclusion, Casey finds herself reconciling with her past and embracing an uncertain future as she learns of her own pregnancy, suggesting that the echoes of her family's trauma may be perpetuated once again. She reflects on the significance of her dreams and the whispering warnings she encountered along the way, hinting at the potential birth of twins as the film closes with the haunting question of what legacy Jumby may bring forth once more.
