NOTE: This spoiler was submitted by Jeremy

1993 – At the Simion Research Hospital, Dr. Florence Weaver (Jacqueline McKenzie) is recording a video log regarding someone named Gabriel and how he has become more unstable. Weaver is then alerted that Gabriel has broken free and is going on a rampage. Several hospital staff members are brutally killed by Gabriel in a locked room. When Weaver and her colleagues are able to enter, they find the corpses of the staff and then manage to grab and detain Gabriel. We see a pair of child’s legs, but when Weaver addresses Gabriel, a glimpse is shown of a grotesque creature screeching at everyone. Gabriel also causes the lights to flicker and uses a radio to project his thoughts. Weaver then declares, “It’s time to cut out the cancer.”

In the present day, Madison Mitchell (Annabelle Wallis) is on her fourth pregnancy after suffering three previous miscarriages. She lives in Seattle with her husband Derek (Jake Abel), who is abusive and useless. Derek blames Madison for the loss of the babies, and during a heated argument, he throws her so hard against the wall that he cracks her head open and leaves a huge hole in the wall. Derek quickly tries to apologize, but Madison locks herself in her bedroom.

Later that night, Madison sleeps while Derek hears a noise in the living room. He finds a shadowy figure appearing to watch TV, but when he turns on the lights, nobody is there anymore. Derek is stalked by the figure until it appears to stab him and then mangle his body. Madison goes downstairs to see what is going on and she finds Derek’s body. The dark figure then appears to chase Madison through the house until she gets knocked unconscious.

Paramedics and authorities come upon the Mitchell house and take Madison to the hospital. She is met by her sister, Sydney Lake (Maddie Hasson), who tells her that she was attacked, Derek is dead, and what’s worse is that she has lost her baby again. Madison wails in agony. Madison’s room is later visited by Detective Kekoa Shaw (George Young), who speaks to Sydney. She tells Shaw that Derek kept her away from Madison, so she never knew how abusive he was to her or that Madison suffered previous miscarriages. Shaw’s partner Detective Regina Moss (Michole Briana White) suspects that Madison killed Derek herself for all the abuse.

Two weeks later, Madison returns home and tries to get her life back to normal. She hears noises and sees what looks like a shadowy figure outside on the street from her window. When Sydney comes to visit later, Madison tells her something that she never knew – before Sydney was born, Madison was adopted by Mr. and Mrs. Lake when she was about nine years old.

An unnamed woman (Jean Louisa Kelly) is acting as a tour guide for a group of people checking an underground exhibit. When the group leaves, the woman is left alone and is stalked by the figure. He attacks her and later brings her to an attic where he keeps her tied up.

At night, Dr. Weaver is in her home where she feels an unnatural presence. Meanwhile, Madison tries to do laundry, but she is startled when she sees what looks like Weaver yelling at her asking why she is in her house. Madison then sees the figure approaching Weaver. Madison remains frozen in place while she witnesses the figure getting closer to Weaver, and through a staticky television, he utters the words – “It’s time to cut out the cancer.” Weaver knows it’s Gabriel, and he proceeds to grab a pointy trophy to stab Weaver to death. Gabriel then takes the sharp end of the trophy and makes his own weapon out of it. Shaw and Moss later go to Weaver’s home and investigate the crime scene, finding a picture of a girl named Emily May and then taking Weaver’s patient records as evidence.

Dr. Victor Fields (Christian Clemenson), a former colleague of Weaver’s, overhears about her death and is mortified by the circumstances. As Fields settles in for bed, he is stalked by Gabriel as well. Madison tries going to bed, but sees Fields in the bed next to her as Gabriel takes his weapon and proceeds to gruesomely stab Fields in the face and neck. Madison and Sydney go to Shaw and Moss with a description of her vision. They find the apartment building, but Shaw and Moss are forced to go knocking on all the doors on the top floor until they finds Fields’s room and discover his corpse. When they go back to the station, Madison tries to identify the killer but can only come up with a monstrous facial description. She goes to the bathroom and gets a phone call from an unknown number. It is Gabriel speaking to her, telling Madison that her real name is Emily. Madison addresses Gabriel by name, meaning she does remember him.

Madison and Sydney go to the home of their mother Jeanne (Susanna Thompson), who becomes mortified when Madison brings up Gabriel. She gives the ladies a videotape of Madison as a child (here played by Mckenna Grace) who appears to be talking to Gabriel as though he were an imaginary friend. Another tape shows Madison on Christmas talking to Gabriel on the phone and asking him not to kill baby Sydney.

Shaw does more research into Weaver’s work and learns of another colleague, Dr. John Gregory (Amil AboulEla), involved in procedures with Gabriel. Sure enough, Madison sees herself in Gregory’s home just before Gabriel gets to him. Shaw finds Gregory’s address and attempts to find him, but he ends up too late and discovers Gregory’s corpse in a bathtub full of blood. Madison still finds herself seeing the scene in the bathroom as Shaw enters, and tries to warn him as Gabriel appears. He nearly kills Shaw but he shoots at Gabriel, who proceeds to flee through the fire escape. Shaw chases him and ends up in a dark alley where Gabriel nearly gets him again, but the villain ends up escaping.

Shaw and Moss meet with Madison and Sydney again, plus a hypnotherapist, as they try to get Madison to remember her childhood connection to Gabriel. They know now that Weaver and her team had Madison, or Emily, as a case study. Madison remembers that Gabriel would speak to her as a child and do bad things that she would get blamed for, like putting his finger through a cake or even going as far as to get Madison to hold a knife and nearly kill her adoptive parents. Madison starts freaking out saying that Gabriel was always making her take the blame, but she is snapped out of it. Moments later, the captured woman breaks free from her bonds, but as she tries walking away, she falls through the floor and lands in Madison’s living room. The woman is taken to the hospital, and Madison is taken into custody.

Shaw and Moss interrogate Madison, where she gets angered at the accusations to the point where she yells and causes all the lights to break. Shaw’s phone then rings, and Gabriel’s voice speaks to taunt them.

Sydney gathers what she can on the Simion Hospital and goes out to find the place is now an abandoned building. She breaks in and manages to find tapes on Emily May. She brings them home so she and Jeanne can watch. They learn that Madison’s biological mother, Serena May (Madison Wolfe), had given up her children to the hospital because she was 15 and they were the product of rape, which Jeanne didn’t know about since the hospital told her that Madison’s mother died. They see Weaver doing a test with Madison as she says that Gabriel speaks to her, and the camera pans over to reveal the truth – Gabriel is a formerly conjoined parasitic twin akin to a teratoma tumor in the back of Madison’s head. Weaver and her team surgically removed Gabriel from her body, but fearing that they might accidentally kill Madison, they took out only as much as they could while the rest of Gabriel remained dormant in her head. The visions that Madison had seen were Gabriel projecting images into her mind that made her think she was living normally in her house, when in reality, he was taking over her body and using it to carry out the murders. When Derek smashes Madison’s head against the wall, that brought Gabriel back out, and explains why Madison always wakes up with her head bleeding after seeing what Gabriel did. As this happens, Shaw learns that the captive tour guide is Serena.

Madison is in a prison cell with other women, who quickly antagonize her, and two of them begin to assault her. Gabriel proceeds to take over, revealing his hideous face from the back of Madison’s head. Gabriel slaughters all the women in the cell and breaks free when a guard comes by to stop him. He rampages through the station and murders other guards, incapacitating Moss, before recovering his weapon.

Sydney meets up with Shaw as they both realize that Gabriel is going after Serena at the hospital. Once they get there, Gabriel begins manipulating the lights and kills one guard by frying his pacemaker. Gabriel makes it to Serena’s room where he hurls his weapon at Shaw’s chest and wounds him, while Sydney tries to stop Gabriel from killing Serena. He hurls a hospital bed at her and appears to crush her as she tries telling Madison that Gabriel was responsible for her miscarriages, as he was feeding off the fetuses to regain power. Gabriel shoots Sydney in the head before going after Serena. She apologizes to Gabriel for giving him up, but he is unmoved and still kills her, saying that it’s her fault that he’s a monster. Then Madison’s voice calls out saying that Gabriel was ALWAYS a monster. It turns out that she has now learned his mind-manipulation skills and forced Gabriel to see what she wants, meaning he didn’t kill Sydney or Serena. Madison regains control of her mind and body and creates a prison for Gabriel to be trapped in, never to allow him to hurt anybody ever again. All he can do is scream in futility from behind bars.

Madison helps take the bed off Sydney and she embraces her, saying that she has always been her true sister.

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A woman named Madison Mitchell has been suffering horrific visions of murders, finding herself at the scene of the crime while frozen in place and being forced to watch. This connects to a figure from her childhood called Gabriel, who appeared to speak to Madison like an imaginary friend and did evil things that she got blamed for.

As a child, Madison was placed in a hospital and was originally named Emily May. Gabriel is actually a parasitic tumor-like twin that was conjoined to the back of Madison's head until he was surgically removed, but with a small piece of him left in case completely removing him may have killed Madison. After Madison's abusive husband Derek smashed her head against a wall, it woke Gabriel up, and he kills Derek plus the doctors involved in removing him from Madison's body. He has been keeping her trapped in mind projections where she believes she is living normally at home while he is actually taking control of her body to carry out the murders.

After slaughtering a whole station of cops, Gabriel goes after his and Madison's biological mother, Serena May. Madison's adoptive sister Sydney and Detective Shaw go to the hospital to stop Gabriel, and while he almost succeeds in killing everybody, Madison manages to play his game and trap him in a mind prison of her own creation, ensuring that Gabriel will remain locked up in her head and unable to hurt anybody. Madison then goes back to help Sydney.