Malignant (2021)

In 1993, Dr. Florence Weaver and her colleagues Victor Fields and John Gregory treat a violent, disturbed patient named Gabriel at Simion Research Hospital. Gabriel, who is able to control electricity and broadcast his thoughts via speakers, escapes and kills several staff members. Weaver determines that he is a lost cause and states the "cancer" must be removed.

27 years later, Madison Mitchell returns to her Seattle home. During an argument about her multiple miscarriages, her abusive husband Derek smashes Madison's head against a wall, drawing blood. Madison locks herself in her room and has a nightmare of a person entering their house and murdering Derek. When Madison wakes up, she finds Derek's corpse before being knocked unconscious by an intruder. Madison awakens in a hospital where her sister Sydney tells her that her unborn baby did not survive. She is interviewed by police detective Kekoa Shaw and his partner Regina Moss, then let go. Madison reveals to Sydney that she was adopted by Sydney's mother at age eight and has no memories of her prior life. As the killer kidnaps a woman running a Seattle Underground tour, Madison's head wound continues bleeding. She has a vision of Weaver being brutally bludgeoned by the killer.

Shaw and Moss discover a photo of Madison as a child in Weaver's house and learn that Weaver specialized in reconstructive surgery. After a vision of Fields' murder, Madison and Sydney approach the police for help. The killer calls Madison, revealing himself to be Gabriel. Madison and Sydney visit their mother and learn that Gabriel was Madison's imaginary friend but may also be someone she knew before her adoption. Shaw finds a link between the doctors and Madison hidden in Weaver's records, leading him to discover the murder of Gregory. The detectives enlist a psychiatric hypnotherapist to unlock Madison's memories. Madison recalls that her birth name is Emily May and that Gabriel almost led her to kill Sydney while Sydney was still in her mother's womb. After Sydney was born, Madison forgot about Gabriel. The woman kidnapped earlier by Gabriel escapes and falls from the attic of Madison's home, revealing that Gabriel was secretly living there. Believing Madison to be the murderer, the police arrest her and place her in a holding cell. The kidnapped woman, Madison's biological mother Serena, is taken to a nearby hospital.

Sydney goes to the now-abandoned Simion Research Hospital and learns that Gabriel is Madison's parasitic twin brother who appeared as a half-formed child facing out of her back. Weaver surgically removed most of Gabriel's body, but could not destroy his brain without killing Madison. She resorted to sealing him inside Madison's cranium, where he remained dormant until the altercation with Derek. Once freed, Gabriel was able to take control of Madison's body to commit the murders. After being attacked by the other inmates in the holding cell, Gabriel takes over Madison's body. He kills the inmates and most of the precinct's officers before going to the hospital where Serena is recovering. He is intercepted by Sydney and Shaw but quickly immobilizes them. As Gabriel attempts to kill Sydney for replacing him in Madison's life, Madison wakes up and takes back control of her body. In a black mindscape, she subconsciously locks an enraged Gabriel behind bars and, after he promises to escape one day, says she will be ready. With his influence gone, Gabriel sinks back inside her head.

Back in the hospital, Madison lifts the bed pinning Sydney and remarks that even though they are not related by blood, she will always love her as a sister. Serena looks on with relief while the electric humming that accompanied Gabriel's attacks is faintly heard.