NOTE: This spoiler was submitted by Evan

The film opens on a woman named Holly (Gabrielle Graham). She sticks a probe into her head. As she turns the dial on the connected machine, her emotions involuntarily range from weeping to laughing. There is a strange gleam in her eyes. Holly disconnects from the machine and puts on her work uniform. She enters a party as a hostess. Holly walks up to a wealthy attorney, grabs a knife, and brutally stabs him in the neck and gut. The partygoers flee in terror and call the cops. Holly revels in the blood before taking out a gun from her purse and sticking it in her mouth. Holly says “pull me out,” and goes to pull the trigger. But her body seemingly revolts and she can’t pull the trigger. Instead, Holly aims the gun at the arriving police and they shoot her dead.

As Holly is shot, the film cuts to an office building. Tasya “Tas” Vos (Andrea Riseborough) is removed from a machine. It is revealed that she is a possessor – an assassin whose consciousness is implanted into the body of another person in order to carry out a hit. The gleam in a person’s eye indicates a person in the film is being controlled by Tas. Tas was controlling Holly’s body and perpetrated the murder (when Holly said “pull me out,” it was actually Tas talking to her support team to restore her consciousness to her real body). To remove a possessor’s consciousness, the host body must be killed.

Tas’ boss Girder (Jennifer Jason Leigh) gives Tas a psychological exam to ensure that Tas suffered no mental damage from possessing Holly. Girder hands Tas various objects and asks her to identify them. Tas passes the test, but doesn’t tell Girder she was unable to make Holly commit suicide. Tas mentions she needs time off to spend with her husband Michael (Rossif Sutherland) and son Ira (Gage Graham-Arbuthnot). Girder seems surprised since Tas told her they were separated and that she thought she was a danger to her family. Tas seems not to remember this conversation.

Tas stands outside her family home and practices what she is going to say and how she will emotionally react to her family (indicating sociopathic behavior). She then enters the home and is warmly embraced by their family. They watch the news (which covers the murder of the attorney). That night, as she and Michael have sex, the memory of her killing the attorney arouses her. She also has visions of Michael bleeding from his neck. Tas calls Girder and asks to come back to work and forego the time off.

The next day, Girder explains she’s getting too old to be a possessor and hoped Tas would take over as the head of the agency. But Girder is worried Tas might be cracking. Girder asks why Tas stabbed the lawyer instead of using the gun they furnished to her. Girder accepts Tas’ explanation.

Girder fills Tas in on their next job, one of the biggest ones the company ever got. They’ve been hired by a man named Reid, who is the stepson of a man named John Parse (Sean Bean). John is the owner of a gigantic corporation that specializes in data mining. Reid wants them to assassinate John and his sister Ava (Tuppence Middleton) so that he can take over the company. Girder is excited because, once they complete the job, their company will effectively be able to control John’s company via blackmailing Reid.

Tas will possess Ava’s boyfriend Colin Tate (Christopher Abbott). Tas does a stake out and becomes familiar with Colin’s mannerisms and speech patterns. The plan is for Tas to spend two days in Colin’s body. She will have Colin express erratic behavior and then murder both Ava and John after a gala John is hosting. Any longer than two days in Colin’s mind and Tas risks permanent damage to her own psyche. We also learn that while possessed, Tas must recalibrate the host’s brain to maintain control over the host’s body (which is what we saw her do with Holly’s body at the beginning). The company abducts Colin and implants a neural network in him.

Tas wakes up in Colin’s body and Ava soon accuses Colin of acting strangely. Colin (under Tas’ control) goes to work at John’s company where he and a co-worker named Eddie (Raoul Bhaneja) discuss an affair Colin is hoping to have with a woman named Reeta (Kaniehtiio Horn). Colin has a low-level job spying on people through their web-cams to mine data about their lives (in this case, Colin is recording the types of curtains each user has). Tas suddenly suffers from psychic backlash while at work. She seemingly regains control and talks Girder (who is monitoring the mission) out of aborting the hit.

The day of the party arrives. Jack is a total asshole who is dismissive of both Ava and Colin (John purposefully has Colin doing menial work at the company to torture him). John eventually belittles Colin in front of the guests. Girder instructs Tas to have Colin get into a public argument to establish motive for the later assassination. Colin is kicked out and then hides in the bushes until the party ends.

Instead of quickly shooting John with the company-provided gun, Colin brutally maims him with a fire poker. Ava walks in on the beating and he shoots and wounds her. Ava writhes in agony before Colin kills her with a shot to the head. Colin then puts the gun in his mouth and asks for them to “pull me out.” Psychic backlash erupts. Colin is unable to pull the trigger. In the assasin offices, Tas pukes up blood. The assassin support staff says that Colin has reasserted control. They want to prematurely disconnect Tas (which would either kill or permanently damage her). Girder orders them to give Tas more time to try and reassert control.

Colin runs to Reeta’s house and takes refuge there. As Reeta showers, Colin babbles to himself. He sees a news report saying that John survived with severe brain damage (but confirms Ava is dead). Colin pulls out the gun and stalks Reeta to the shower before blacking out. While Colin is unconscious, Tas imagines herself visiting a wheelchair-bound and scarred John.

Colin awakens and hears a knock on the door and is surprised to see Eddie there. Eddie enters the apartment and knocks out Colin. When Colin awakens, he is connected to a recalibration machine and Tas is seemingly back in control. It turns out Eddie works for the assasination company and was planted as a lifeline for Tas in case anything went wrong. Eddie explains that Tas needs to complete the suicide (so that the company’s cover isn’t blown) and they can pull her out. Tas says she still isn’t fully in control. Eddie says he will help her override Colin’s consciousness and they begin a process to do so.

During the process, we see Colin’s consciousness enter the room and attack Tas’. Colin rips off Tas’ face, wears it as a mask, and is able to access her memories. Then Colin/Tas blacks out. When Colin awakens (with Colin seemingly in control again), Colin finds Eddie dead on the floor. He also finds Reeta dead from their earlier encounter. Colin’s hand suddenly grabs the gun and fires it at himself, but Colin dodges the bullet. He then angrily leaves Reeta’s house.

Colin tracks down Tas’ house based on the memories he experienced. He breaks and holds Michael hostage. Colin demands that Tas come out and explain what is happening. He and Tas’ consciousness then talk on the psychic plane. Tas says that Colin has been in control for some time (hinting that he is responsible for some of the prior murders). Colin threatens he will kill Michael if she doesn’t help him. Although Tas says she “loved” Michael, she encourages Colin to kill him.

While Colin is distracted during this psychic conversation, Michael disarms him. Before he can shoot Colin, Colin butchers him to death with a meat cleaver (it is unclear who is now in control of Colin’s body). Tas then asks to be pulled out, but again can’t make Colin commit suicide. Suddenly, Colin is stabbed in the throat by Ira. As Colin bleeds out, he shoots Ira dead (it is intimated that Tas is in control of Colin when this happens). As Ira and Colin die, Colin hears Ira say “pull me out.”

The film cuts to the assasination offices. Tas is removed from the machine connecting her to Colin. And Girder is removed from the machine connecting her to Ira.

The film ends with Girder giving the psychiatric evaluation to Tas. The state of Tas’ mind is left ambiguous (and may even suggest Colin’s psyche is in Tas’ body).

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Tas works for a company whereby her consciousness is implanted in other peoples' bodies in order to carry out assassinations. She loses control on one assignment and the host consciousness, Colin, attacks and murders Tas' family (although it is unclear if Tas - who is implied to be a sociopath - was in control during the actual murders). Tas' boss (possessing Tas' son before he is murdered) then kills Colin, allowing them to extract Tas' consciousness from Colin's body. But the film ends ambiguously as to the state of Tas' psyche.