The story unfolds within the home of the Martin family, introducing us to the father, Russell (Jay Duplass), and his young daughter, Isabela, known as "Bela" (Arabella Olivia Clark). During a playful moment between father and daughter, the mother, Ursula (Jessica Chastain), intervenes to tell Bela it is time for bed. However, once her parents depart the room, Bela whispers an invitation for her "Other Mommy" to appear so they can continue playing. As she approaches her closet door, a mysterious, golden flash of light illuminates her face, hinting at a supernatural presence.
The following day, while Ursula leaves for work and Russell takes a nap, Bela spends her time preparing a project for her new school. Suddenly, a figure appears and begins cooking pasta for the child, but the entity is clumsy, adding barely any water to the pot, which causes the food to burn. This entity speaks to Bela, claiming she was once a part of the girl's heart, while a single tear falls down her cheek. Realizing this is the Other Mommy, Bela becomes frightened and flees the room. Russell soon notices the smoke from the kitchen, and after extinguishing the fire in the sink, he is horrified to discover the body of the Other Mommy twisted into a grotesque, unnatural shape beneath the table. As the entity begins to float, Russell and Bela flee the house in terror.
Following the arrival of police and medics, the real Ursula returns home. Although she attempts to comfort her daughter, Bela senses a strange disconnect from her mother and notices a growing strain in her parents' marriage. After being tucked into bed, Bela is once again tempted by the Other Mommy, who uses the glowing light in the closet to lure her.
Later, Ursula and Russell host a party. While the guests mingle, Bela is left upstairs under the care of a babysitter named Calvin (Sean Kaufman). Among the guests is a man named Frank (Arian Moayed); Bela actively avoids him, as she is aware that Frank and Ursula have been engaged in a secret affair. In an attempt to prove the existence of the Other Mommy, Bela shows Calvin the closet, and he is briefly terrified after seeing the creature crawling across the ceiling. Once Calvin leaves the room, the entity shapeshifts to take his form and tries to manipulate Bela into letting her into her heart. Bela resists and punches the creature, causing the Calvin disguise to rip apart. The entity then causes Bela to crash through the ceiling and land right in the middle of the party. Before Russell can intervene, the Other Mommy makes her terrifying presence visible to Ursula.
Too frightened to remain in their own home, the Martins seek refuge in Frank's high-security residence. Despite the lockdown, the Other Mommy finds a way to reach Bela. The girl, feeling resentment, asks the entity to make Frank "go away." The Other Mommy complies by posing as Ursula to deceive Frank, only to brutally drive her hand through his chin and out of his mouth. The family watches in horror as the entity tosses Frank around; despite Bela's attempt to kick the gun back to him, the creature uses its powers to bend Frank's body until he violently explodes.
The traumatized family retreats to a children's restaurant to discuss their next move, considering a trip to the home of Russell's mother, though Ursula is hesitant because the woman is deeply invested in the supernatural. While at the restaurant, Bela plays in a ball pit with another girl, but she is lured into a trap. The Other Mommy warps reality, causing Bela to sink into the balls and teleporting her to a surreal ocean. There, the entity tells Bela that Ursula never actually wanted her, while a mansion appears on a distant hill. Meanwhile, Russell desperately searches for his daughter, knowing the entity is near. Eventually, the entity teleports Bela and several plastic balls back into the family car, where her parents find her.
With no other options, they travel to the home of Grandma Ruth (Karen Allen), who intends to call her medium contacts to expel the entity. Before the professionals arrive, Ruth shares a vital lesson with Bela about knowing who to let into your heart and, more importantly, who to keep out.
A team of ghost researchers and exorcists, led by Lois Anthony (Dichen Lachman), arrives to investigate. They determine that the Other Mommy is actually a manipulative demon targeting Bela because of her purity; if the demon can enter her heart, it will gain a physical body to live in. They suggest that removing Bela's purity is the only way to make the demon leave. The team establishes a perimeter of salt and begins a seance. When the Other Mommy reveals herself, Lois, driven by fascination, touches the creature's skin and gets a glittery gold substance on her fingers. This substance causes Lois to gruesomely melt, and the demon proceeds to slaughter the rest of the research team.
Forced back to their own home with nowhere left to run, the Martins arm themselves for a final confrontation. The Other Mommy attacks, driving them all into the basement. There, the entity forces Ursula to reveal a painful truth: Russell is not Bela's biological father. Ursula had been pregnant from a previous marriage before meeting Russell. While Russell was happy to step in as a father, Ursula confesses that she was not ready for motherhood and initially feared that having Bela meant giving up her freedom, though she insists she loves her daughter now. Heartbroken and angry, Bela runs away from her parents.
The Other Mommy finds Bela locked in the bathroom and disguises herself as the girl. Bela nearly gives in and allows the entity into her heart, but she suddenly recalls Grandma Ruth's words: that parents who promise their children they will never get hurt are lying. Because the Other Mommy promises Bela she will never be hurt, Bela recognizes the lie. In a moment of strength, she tears off the entity's skin and rejects her entirely, banishing the demon for good.
Some time later, as the family puts up Christmas decorations, a sense of peace has returned, though Bela notices that Ursula still seems to be hiding feelings of deep unhappiness.
