At the Somafree Institute, Dr. Hal Raglan humiliates his patient, Mike, by stating, > "you're just a weak person. You must have got that from your mother. It probably would have been better for you if you had been born a girl!" On a dimly lit stage, Raglan demands that Mike show his anger, revealing angry red blotches across his torso that leave the audience in shock. Raglan explains this phenomenon as psychoplasmics, which refers to the physical manifestation of one's mental rage.
Meanwhile, Frank Carveth is picking up his nine-year-old daughter, Candy, from a 'Private Guest Room'. Dressed in a red coat with fur trim, Candy’s back is discovered to have bruises and scratches during a bath. Frank immediately drives to the Somafree Institute to confront Raglan, demanding to see his wife, Nola, who has been committed there after experiencing a nervous breakdown. Raglan refuses to allow Frank to see Nola, claiming legal issues if he denies Nola a crucial part of her treatment. Frustrated, Frank seeks advice from his lawyer, Al Resnikoff, who informs him that Nola holds a stronger legal position because of her commitment. Determined, Frank takes Candy to her maternal grandmother, Juliana, who appears quite anxious.
Inside the institute, Raglan attempts to get information from Nola by pretending to be Candy. In response to her distress, he states, > "Mummies don't hurt their own children." Nola sobs that, indeed, they do, mentioning her own troubled childhood. Raglan encourages Nola to embrace her anger, saying, "Go all the way through it, right to the end."
That evening, while investigating a noise in the kitchen, Juliana is attacked and killed by what appears to be a small child in a red hooded coat, leaving Candy as a quiet, horrified witness. The next morning, Frank learns of Juliana's murder and is advised by police psychologist Dr. Birkin to guide Candy’s recollection of the event, as repressed memories can lead to breakdowns. Dr. Birkin warns him, "These things tend to express themselves in one way or another."
Back at the institute, Raglan, posing as Candy's father, stirs up anger in Nola, creating red welts on her forehead while they delve into her troubled emotions. At the airport, Frank meets Barton Kelly, Nola's estranged father, who attends the funeral of his ex-wife. On the advice of Resnikoff, Frank contacts Jan Hartog, an ex-patient of Somafree seeking legal action against Raglan, who expresses his struggles with the treatment: "Raglan encourages my body to revolt against me. And it did. I have a small revolution on my hands and I'm not putting it down very successfully."
Back at the institute, a distraught Barton confronts Raglan, demanding to see Nola. Simultaneously, Frank attends Candy's school, where her teacher Ruth Mayer joins them for dinner. That night, as Barton finds himself alone, he is brutally murdered by the hooded creature, furthering the cycle of violence tied to Nola's rage.
When Ruth innocently receives a phone call from Nola, her jealousy erupts into chaos. At the police station, Frank learns from the pathologist that the creature that attacked him perished because it "ran out of gas," resembling nothing human with its peculiar features. Meanwhile, outside Nola’s room, Raglan deceives her with claims that Frank will leave her for Ruth.
Upon returning home, Frank finds Candy terrified after a nightmare and reassures her that the creature is dead. The following day, Raglan learns of Barton’s murder and prepares to evacuate all patients from the institute. Frank, discovering Mike as a fellow patient of Hartog's, grows increasingly angry when Mike complains about being abandoned.
In class, Candy is taken captive by two small creatures who brutally attack Ruth, leading to a frantic search by Frank who arrives too late. As the horror unfolds at the institute, Nola becomes aware of her children’s growing threats.
In a heart-wrenching climax, Frank discovers the shocking truth about Nola, who has physically manifested her rage into the form of these Brood children, who do her sinister bidding whenever her anger is stirred. Raglan explains to Frank that to retrieve Candy, he must convince Nola to suppress her rage. Frank discovers Nola in a trance, revealing a grotesque external womb connected to her body that nurtures the Brood.
As Frank intervenes in Nola’s rage-fueled birth, the Brood awaken, launching a deadly assault on Raglan. In a moment of desperation, he strangles Nola to save Candy, severing the connection that animated the creature children. As silence descends upon the institute, Frank finds Candy trembling in fear, realizing with horror that her marks may signify a deeper connection to her mother’s rage. With a blank expression, Candy is driven home by Frank, unaware of the dark legacy she carries within.
