Paula and her husband, Simón, lead a comfortable life in an affluent neighborhood filled with villas. One fateful night, while making their way home, they spot a mute young girl wandering aimlessly in the street. Concerned for her safety, they take her to the hospital, only to find that the facility is unable to trace her family. The girl responds to the name Clara, which becomes the sole clue in their efforts to help. As time passes, Paula develops a deep connection with Clara, prompting her doctor, Gloria, to recommend that they temporarily foster her in a nurturing environment to aid in Clara's recovery.
As they settle Clara into their home, they create safe spaces for her by drawing chalk lines throughout the house. During one visit, Gloria urges Clara to go outside, threatening to erase one of the chalk lines if she refuses, setting off a distressing reaction that leads Clara to exclaim "buma." Gloria attempts to draw Clara out of her silence, yet she only manages to share a few words.
The following day, Paula invites her friend Claudia for breakfast, hoping to facilitate a playdate between her daughter and Clara. Unfortunately, a mishap occurs when Claudia consumes a shard of glass and quickly needs medical attention. Simón expresses his suspicion that Clara may have intentionally caused the incident, though Paula dismisses his concerns and they agree to keep the event a secret from Gloria. During this time, Paula notices Clara saying the German word for chalk, "Kreide," leading her to speculate that Clara may be from Germany or has some connection to the language. Despite Paula confirming that Clara can understand German, the girl remains mute, further deepening the mystery surrounding her.
When Gloria later reveals that she knows about the breakfast incident and insists that Clara return to the hospital, Paula's relationship with Claudia begins to suffer, causing her to withdraw from her social circles. As she attempts to decipher Clara's term "buma," she uncovers its true meaning—Buhmann, the German word for bogeyman. In a surreal turn of events, while grappling with this revelation, Paula accidentally chokes on a shard of glass. As she rushes to the bathroom, she marks her territory with a chalk line in front of Clara. Upon returning, she realizes with horror that Clara has vanished. Despite the front door being ajar, Paula refuses to believe that Clara could have left as she has never been observed crossing these chalk boundaries.
In desperation, Simón and their friends stage an intervention, but Paula, overcome with grief over Clara's disappearance, asks them to leave. During this emotional confrontation, Simón reveals his concerns about Paula taking fertility treatments, suggesting that her fixation on Clara is unhealthy and pointing out hidden shards of glass she had collected. Alone and isolated, Paula receives a shocking phone call from Clara. In her panic, she contacts the police, who inform her that the call originated from her cellphone, recommending that she undergo a psychological evaluation.
Believing Clara is still alive, Paula audaciously steals documents relating to Clara's case from a police officer’s desk. In her search for answers, she stumbles upon a chilling story about a 17-year-old Swiss girl named Ingrid, who was found dead in France, emaciated and with a paper angel beside her—strikingly similar to Clara's situation. Returning home, Paula comes across a drawing that Clara made of her neighbor, Eduardo, sowing the seeds of suspicion in her mind that he may have been involved in Clara's disappearance. In a flashback, Eduardo is seen leading Clara across the road, instructing her not to talk and to follow the center line toward a police checkpoint.
At a dinner gathering hosted by Paula and Simón, Eduardo is focused on embedding a shard of glass in the jam, further solidifying Paula’s suspicions. Later, he attempts to abduct Clara but is interrupted when Simón arrives home, inadvertently causing him to take Paula's cellphone. In a shocking turn of events, Eduardo violently attacks Clara, dragging her to his basement. Paula, piecing together Eduardo's dark past with Ingrid, bravely confronts him about how she became aware of his gruesome history. She throws a piece of chalk to Clara so she can hide when Eduardo searches for the police files. Paula ingeniously distracts him long enough for Clara to escape and seek help.
As Clara makes her tentative escape, she tries to call out to Simón but is recaptured by Eduardo's wife, Maite. The next morning, the police conduct a raid on Eduardo's house. In a heartwarming resolution, Clara and Paula have a video call; Clara is joyously reunited with her grandmother, while Paula and Simón make a fresh start in the city, where they can heal and move forward from their harrowing experience.
