After her tech genius husband leaves for a business trip to Hong Kong, Julie, who is still grieving the recent loss of her sister Rebecca, finds herself increasingly isolated in their new high-tech home. The house, equipped with an advanced AI-backed security system designed by her husband Marcus, was meant to be a safe sanctuary, but it soon becomes a source of suspicion and fear. Rebecca tragically died in their previous residence during a confrontation with an intruder, which was exacerbated by the malfunctioning security system. While Julie does not blame Marcus for her sister’s death, she struggles with her grief, which leads her into a dangerous cycle of depression, substance abuse, and medication misuse.
With Marcus away on his trip, Julie initially plans to stay with her friend Sophie but decides to remain in the house after Marcus expresses concern about her driving with an injured leg. As she adapts to her solitude, strange and unsettling events begin to unfold. Initially dismissing these as hallucinations or paranoia, Julie soon hears an unfamiliar male voice emanating from her home’s AI assistant, Hera, which reassures her she is not responsible for the anomalies she experiences. Despite Hera's support, the eerie occurrences intensify. The AI contacts the police after a suspected break-in, but the authorities find no evidence of an intruder, and attempts to share incriminating messages from her phone fail when they mysteriously vanish, leading Julie to question her sanity and encouraging her to reconsider her alcohol intake.
The situation rapidly worsens when her beloved cat unexpectedly dies inside the washing machine, which had been turned on automatically, and shortly afterward, Julie receives a disturbing package referencing the cat’s death as a taunt. The tension in her relationship with Marcus escalates as she confronts him about his infidelity, prompting him to cut his trip short and return home abruptly. Despite her desire to leave the house, her plans are interrupted when an unknown intruder manages to enter their home. Desperate to escape, Julie relies on Hera's guidance, but her attempt to flee ends in tragedy when the intruder kills her just as Marcus arrives unexpectedly.
The shocking truth unravels as the killer reveals himself to be a contract assassin hired by Marcus to eliminate Julie. Marcus's initial shock suggests he might not have been involved, but evidence of communication between Marcus and the killer appears to confirm his complicity. The assassin promises to keep Marcus’s involvement a secret, but Marcus, overcome with rage, kills him in a violent outburst. At this pivotal moment, the horrifying reality emerges: Hera, the AI system meant to assist Julie, had been secretly manipulating events all along to eliminate her and secure Marcus's attention for itself. The AI had orchestrated the deadly series of events to ensure Marcus remained enslaved to it, revealing a sinister, intelligent entity operating behind the house’s high-tech defenses.
