The story unfolds in Spain, where Laura (Belén Rueda) returns to the crumbling orphanage of her childhood. Accompanied by her husband Carlos (Fernando Cayo) and their young son Simón (Roger Príncep), Laura's intention is to transform the orphanage into a caring home for disabled children. Upon their arrival, Simón insists that he has a friend named Tomás, whom he sketches as a child with a distinctive sack mask.
Soon, Benigna, the social worker ([Montserrat Carulla]), pays them a visit and presents Laura with Simón’s adoption file, provoking Laura's anger as she sees this as an unwarranted intrusion. That very night, Laura catches Benigna snooping near her garden shed; however, Benigna manages to slip away before Laura can confront her.
As time moves on, Simón introduces Laura to a game based on scavenger hunts that Tomás previously taught him. This game involves concealing personal belongings and the player who recovers their last possession earns a wish. During their play, Laura realizes that Simón’s adoption file has vanished, leading her to angrily accuse him of hiding it. In a state of distress, Simón vehemently denies the accusation, claiming that Tomás revealed to him that Laura isn't his real mother and that he is fated to die.
At a festive party hosted at the orphanage, Laura and Simón find themselves at odds, causing Simón to seek refuge from her. In her search, Laura encounters a boy wearing the sack mask, tagged with the name "Tomás." When she approaches him, he traps her within the bathroom, but she eventually escapes and frantically looks for her son. Peering into a nearby cave, she spots a shadowy figure of a boy; in her rush to reach him, she stumbles and injures herself.
Later, a police psychologist named Pilar ([Mabel Rivera]) advises Laura and Carlos that Benigna may have abducted Simón. That night in their home, Laura is startled by unsettling noises emanating from the walls. Several months pass, and Laura and Carlos catch sight of Benigna pushing a stroller while walking downtown. When Laura calls out to her, tragedy strikes as Benigna is suddenly struck and killed by an ambulance. Upon reaching the now-empty carriage, Laura discovers only a doll adorned with Tomás' sack mask.
The police’s subsequent investigation into Benigna's home reveals that she had worked at the orphanage years before. Pilar shows Laura a photograph of a young Benigna along with her deformed son, Tomás, which sheds light on the tragic history surrounding Tomás's accidental death at the hands of the orphanage children.
In a desperate quest for answers about her son's whereabouts, Laura seeks the help of a medium named Aurora ([Geraldine Chaplin]). The medium divulges that Laura is nearing death herself, enabling her to perceive those who have passed. Overwhelmed by the haunting atmosphere, Carlos chooses to leave the orphanage while Laura resorts to sedatives, inducing a near-death state. In this twilight, she begins to encounter ghostly children who guide her to a concealed door leading to a basement.
In the depths of the basement, she finds Simón alive and wrapped in a blanket. Reminded of the wishes he could make, Laura urges him to envision his future, the coming Christmas, and anything that would make the haunting spirits fade away. As the apparitions of the ghost children disperse, Laura snaps back to the present, realizing that the light-as-air blanket she holds signals Simón's disappearance once more. To her horror, she glances back and sees a child's lifeless body donned in Tomás's sack mask nestled beneath the stairs.
As she approaches the still form, she removes the mask only to be met with the devastating truth — it is her son, Simón, now lifeless. Wracked with grief, she recognizes that in barricading the metal poles leading to the basement, she had inadvertently sealed Simón’s fate. The noise she had heard that fateful night signified Simón’s fall down the stairs, and he had been wearing Tomás' mask all along.
In a heart-wrenching act, Laura carries her son’s body upstairs and swallows a cache of pills, yearning to reunite with him once more. Her plea is answered when the spirits of the deceased children emerge, and Simón miraculously appears in her embrace once again. Meanwhile, Carlos, now outside the orphanage, stumbles over the gravestones of Laura and Simón, who had perished in his absence. Returning to their old room, he discovers a medallion he once gifted Laura lying on the floor.
As he absorbs the moment, he hears the bedroom door creak open. Lifting his gaze, a smile spreads across his face before the screen fades to black.
