A woman named Holly sticks a probe into the top of her head and turns the dial button on a machine. She goes to a party and brutally stabs a wealthy attorney. She takes out a gun and puts it in her mouth, but then aims it at police, and they shoot her dead. Afterward, a woman named Tasya Vos is removed from a machine. She is a “possessor”: a contract killer whose consciousness is implanted into the body of another person in order to carry out an assassination. Tasya was controlling Holly’s body and committed the murder, but couldn’t overcome Holly’s resistance to committing suicide.

During a post-assassination debriefing by her boss Girder, Tasya identifies objects of her past to prove that her sense of self is intact. One of the objects is a mounted butterfly, which she expresses regret for killing as a child. She wants time to reconnect with her estranged husband Michael and son Ira. When she visits them, she must rehearse how to engage in ordinary conversation, showing she is unsteady in civilian life. Tasya’s next job is one of the biggest ones the company has ever received. They’ve been hired by Reid, the stepson of John Parse. John is the owner of a massive corporation. Reid wants them to assassinate John and his daughter Ava so that he can take over the company. Girder is excited because, once the job is done, the company will be able to control John’s company via blackmailing Reid.

Tasya is instructed to possess Ava’s boyfriend Colin Tate. The company abducts him and implants a neural network in him. Tasya wakes up in Colin’s body, and lives his life, finding herself attracted to Ava. She is blindsided by interactions with Eddie, a co-worker at Parse’s company, and Reeta, a friend of Ava’s whom Colin has slept with before. On the day of a party, Girder instructs Tasya to have Colin get into a public argument with Parse to establish a motive for the later assassination. After doing this, Tasya waits till the end of the party. Colin attacks John with a poker instead of the company-provided gun. He shoots and kills Ava but is unable to commit suicide. Back at the headquarters, Tasya vomits up blood while hooked into the machine and the staff says that Colin has reasserted control. They want to disconnect her, but Girder orders them to give her more time to reassert control.

Colin runs to Reeta’s home, where Eddie arrives and knocks him out. When Colin awakens, he is connected to a recalibration machine and Tasya is seemingly back in control. Eddie was working for the assassination company the entire time and was planted as a lifeline for Tasya in case anything went wrong. He explains that she needs to complete the suicide and then they can pull her out, but she says she isn’t fully in control. Eddie says he will help her override Colin’s consciousness. News reports reveal that John Parse survived the failed assassination.

During the overriding, Colin’s consciousness enters the room and attacks Tasya. He rips off her face, wears it as a mask, and accesses her memories. Their combined form blacks out. When Colin awakens, he finds Eddie and Reeta dead on the floor. He tracks down Tasya’s house based on her memories, and holds Michael hostage. He demands that Tasya explain what is happening. He and Tasya’s consciousness talk on the psychic plane. Colin threatens to kill Michael if she doesn’t help him but she actually encourages Colin to kill him.

Michael disarms Colin but he butchers Michael to death with a meat cleaver. Tasya asks to be pulled out, but can’t make Colin commit suicide. Suddenly, Colin is stabbed in the throat by her son Ira. As he bleeds out, he shoots Ira dead; before he expires, Ira also asks to be pulled out, indicating someone else has been controlling him. Tasya is removed from the machine connecting her to Colin, while Girder is revealed as Ira’s possessor, also removed from her machine. This time, during the debriefing, Tasya remembers killing the butterfly, but does not express remorse, suggesting she has lost the empathy she previously had.