In an alternate version of 2008, Tasya Vos, portrayed by Andrea Riseborough, is a skilled assassin with a unique method: she takes control of other people's bodies to execute her assignments. Utilizing a specialized machine that connects to her victims via an implant in their brains, Vos can insert her consciousness into their minds. The grim aspect of her work entails that she must force her host to commit suicide at the conclusion of each mission, allowing her to return to her own body.
As Vos immerses herself in the identities of others, she begins to lose touch with her own self. The lines blur between her professional life and her personal relationships, particularly with her husband, Michael Vos, and their son, Ira. With violence infiltrating her everyday moments—whether tucking Ira into bed or being intimate with Michael—her mental state deteriorates.
Her handler, the retired hitman Girder, played by Jennifer Jason Leigh, admonishes her for wanting to maintain connections with her family. He believes that emotional ties hinder her effectiveness as an assassin. In a debriefing meant to restore her sense of self, Vos reflects on various mementos from her past and lingers on a butterfly she once pinned and framed as a child, confessing her guilt over its death.
Despite her waning mental stability, Vos accepts a high-stakes job targeting wealthy CEO John Parse, depicted by Sean Bean, and his daughter Ava, played by Tuppence Middleton. The plan involves taking over Ava's fiancé, Colin Tate, represented by Christopher Abbott. Although the mission sees Ava lose her life, Parse survives—a partial victory for Vos.
In a desperate attempt to unbind from Tate's body, Vos tries to make him turn the gun on himself; however, he stabs himself in rebellion, damaging the implant in the process. As the events unfold, Tate regains control over his own consciousness and is disoriented by intrusive memories of Vos's life, leading him to flee to his friend Reeta's apartment, ultimately killing her in his turbulent state.
The chaos only escalates when Eddie, another associate of Vos, arrives to assist in regaining control. Even with Eddie's help, Vos finds herself unable to make Tate end his life. A psychic battle ensues, with Tate's will overpowering Vos's, granting him access to fragments of her memories, including intimate details of her family life. In a tragic turn, he kills Eddie before confronting Vos at her home.
Tate holds Michael at gunpoint, demanding answers about what Vos has done to him. In a struggle, Michael is fatally wounded with a cleaver, and as chaos reigns, Ira, desperate to save his mother, stabs Tate before succumbing to a gunshot from Tate himself.
Finally, Vos returns to her own body, only to discover that Girder had taken drastic measures by controlling Ira to end her entanglement with Tate. With her family now gone, she finds herself devoid of any human attachments, achieving the emotional detachment that Girder desired for her.
In the concluding debriefing, Vos revisits the same memories from earlier, handling the butterfly once again. This time, however, she exhibits no remorse for taking a life.
