Rachel and John have just welcomed their newborn son, Eli, and get settled into their new apartment along with John's beloved childhood pet cockatoo.Prior to Eli's birth, Rachel's sister Vivian was hospitalized after the death of her own infant son Zachary, so their mother sent Zachary's old things for use by Rachel's new baby. While going through the boxes, Rachel discovers an old Hebrew children's book, and explores it with John (who has learned Hebrew following his religious conversion), and after finding the tune, Rachel sings it to Eli to put him to sleep.Shortly after, Rachel and John both begin to experience vivid hallucinations of an old hag. At one point, the apartment loses power, and Rachel runs out of the apartment following disturbances by the hag, insisting there is an intruder, and John investigates. Armed in the dark with a fireplace poker, John is startled by his cockatoo flying past him, and strikes it out of the air, killing it. John grieves, as it is a pet he has had since he was 11 years old. The incident culminates a series of small frustrations that begins to drive a wedge between Rachel and John.Finally, Rachel elects to visit Vivian in the hospital to try to get an explanation for the book. Vivian is shaken to hear Rachel has discovered the book, as she instructed her mother to discard it. Vivian explains to Rachel that the lullaby is a conjure song for Lilith, a demon in Jewish folklore who is said to be Adam's first wife in the Bible. According to the lore, Lilith was cast out of the Garden of Eden for her unwillingness to submit as Adam's wife. Wandering the earth alone, she gave birth to flawed and grotesque babies, as Adam and his new wife Eve gave birth to perfect ones - humans. In vengeance and jealousy, she becomes a demon, and spirits away the human babies of all those who sing her lullaby - a song initially sung to her by Adam, which she twisted into a summoning spell. Vivian tells Rachel that her son Zachary was not killed, but taken by Lilith, and they urgently must return to the apartment to protect Eli.Meanwhile, John visits the synagogue to receives help from the Rabbi. She informs him that Vivian came to her with the same concerns, but there is nothing she can do to help. Shortly after, John receives a mysterious phone call instructing him to come to a local address, where he will receive the answers he seeks. John arrives at the sequestered practice of Rabbi Cohen, who is known for performing Jewish services that will not be honored by the greater church, due to their taboo or unorthodox nature. John brings the book to Rabbi Cohen, and hears the story of Lilith. Rabbi Cohen explains that the hag haunting John and Rachel is a minion of Lilith known as the Crone. She is the first in a series of agents dispatched by Lilith to claim Eli. The next set of agents will be Lilith's own young, the Lilim - the grotesque, distorted babies birthed after her banishment. John is also informed that the accidental killing of his cockatoo was a deliberate part of Lilith's summoning ritual, which is the slaughter of a beast considered "unclean" to Judaism. This leaves the final ritual, which is the spilling of the blood of the infant to be taken by Lilith. If Eli sustains a bloody injury, the ritual will finalize, and he will be abducted and replaced with a dead golem.Rabbi Cohen sends John home with instructions on dispelling Lilith's agents, but does not allow him to leave with the lullaby book.Rachel and Vivian are the first to return to the apartment. Vivian covers the mirrors, warning that they are portals. Overwhelmed, Rachel hands Eli off to Vivian to step to the bathroom and recuperate. While there, she overhears Vivian singing Lilith's lullaby to Eli.Rachel carefully steps back into the room, finding Vivian in a chair cradling Eli, happily singing the lullaby. Vivian tells her that her plan is to prick Eli's finger to summon Lilith, and will follow behind Lilith to her realm to reclaim Zachary. She also reveals she never instructed her mother to discard the book, as she fully intended it to be found by Rachel. Vivian urges Rachel's trust, as this will ensure both Eli and Zachary are returned safely. Rachel feigns cooperation long enough to retrieve Eli from Vivian's arms and return him to his cradle. Rachel then attacks Vivian, and the two struggle in the hallway. Ultimately, an agent of Lilith arrives to cut Eli, and he is taken through a nearby standing mirror and replaced with a golem. Rachel stares down hopelessly at the golem in Eli's crib, until she turns to face the mirror and is pulled through by Vivian, who has already walked through.Rachel arrives in the first area of Lilith's realm, on the other side of the mirror, where she is able to see into Eli's room as though a window. Rachel nearly sinks into a pile of animated golems, until she is saved by Vivian, who pulls her along to continue.They traverse a series of hallways, all populated by wall mirrors where she can see into the bedrooms of other couples with infant children, via the mirrors in their own rooms. They arrive at the final room, a room full of cribs containing the babies successfully taken by Lilith. As Rachel clamors to each crib looking for Eli, Vivian seeks Zachary.Rachel successfully finds Eli, but is caught by Lilith, who throws her back and reclaims Eli, placing him back in the crib. Rachel lunges at Lilith, but is grabbed, and Lilith begins to sing her lullaby to Rachel, slowly draining the life from her. Before Lilith can kill Rachel, she is ambushed by Vivian, who thrusts Eli into Rachel's arms and instructs her to run. Vivian apologizes for what she's done, and sacrifices herself as a distraction for Rachel's escape, as Lilith sings to her and drains her life away. Rachel flees through the mirrored hallways, pursued by Lilith, looking for the mirror in her own home.Meanwhile, in the apartment, John has arrived to discover the golem. Rachel locates the living room mirror and bangs on the mirror to let John know she is on the other side. Per the instructions of Rabbi Cohen, John has lit a candle to expel Lilith's agents, and is reciting the Hebrew prayers to close the portal to Lilith's world. John's prayers successfully soften the portal, and Rachel breaks through with Eli, shattering the mirror and closing the portal.John and Rachel embrace on the floor in their success, and Rachel checks Eli, bundled up in his blankets. Rachel discovers a birthmark that Eli did not have, and realizes Vivian secretly gave her Zachary instead of Eli. Vivian rescued Zachary, leaving Eli imprisoned in Lilith's realm.Some time passes, and Zachary is now a walking toddler. One of Rabbi Cohen's men meets John in a park and hands over the book. He confirms that Rabbi Cohen does not know the book is missing. John returns with the book to Rachel, who sits with Zachary. Exchanging few words, they ponder their next move.