Beth finds herself grappling with profound grief after the tragic suicide of her husband, Owen. As she drowns her sorrow in alcohol and sifts through Owen's belongings, she tries to present a facade of stability despite the growing concern from her friend Claire and neighbor Mel. One particularly perplexing detail is Owen's chilling suicide note, which reads: >"You were right. There is nothing. Nothing is after you. You're safe now." This message leads to a series of unnerving supernatural occurrences in the house, alongside the discovery of an unusual reversed floor plan.
The tension escalates when Beth stumbles upon a photo on Owen's phone featuring a woman who bears a striking resemblance to herself. This revelation leads her to question whether Owen may have been unfaithful. During an outing with Claire, a drunken moment compels Beth to confess that she had a near-death experience following a car accident years ago, where she concluded there was nothing after death—a belief that ultimately caused friction between her and Owen as he sought to challenge her perspective.
Haunted by her sense of loss and guilt, Beth recalls how, during her own struggles with depression a year before, Owen had begun to sleepwalk, suggesting an emotional transference that she feared impacted him. One fateful night, she is stirred from sleep by a spectral presence and witnesses a disturbing scene of terrified women fleeing through the woods to jump into a lake. Blood on the boat where Owen took his life intensifies her feelings of dread as she senses an invisible entity in the house.
Determined to unveil the source of the eerie lights she sees across the lake, Beth discovers a mirror image of her home, where she encounters ghostly apparitions of women intertwined with Owen's presence. After losing consciousness, she awakens back in her own home, driven to find the reversed house once more. This time, however, it appears unfinished and vacant. While rummaging through the space, she retrieves a peculiar statue, later confronting Mel about the house, only to hear his claim that he never witnessed it—though he once saw Owen in the woods with a woman resembling Beth.
In her quest for truth, Beth examines Owen's laptop, unveiling a collection of photos of various women, many of whom share her appearance. She identifies the statue as a voodoo doll linked to the occult, realizing that Owen was likely investigating ways to ensnare demonic beings. Visiting the bookstore where Owen acquired his materials, she encounters Madelyne, one of the women from the photographs, who firmly denies any affair with Owen. Concerned for her wellbeing, Claire urges Beth to leave the house for a few days.
Upon returning to gather her belongings, Beth confronts the ghostly presence within the house. Madelyne arrives and recounts how Owen had invited her to the mirroring dwelling. However, a terrifying episode unfolds when Owen's threatening kiss escalates into an attempt to choke her. A disoriented Beth later discovers the bodies of the women highlighted in Owen’s photos hidden beneath the floorboards.
An invisible force begins to embrace Beth, and in a moment of desperation, she clings to it, misinterpreting it as Owen’s spirit. This entity reveals its true nature, disclosing the chilling reality of Owen’s actions and his attempts to appease it by killing women who looked like Beth. The entity shares that it is what Beth perceived during her near-death experience, calling itself "Nothing." It sought to seduce Owen into killing Beth to reclaim her, but he resisted and instead constructed the reversed house, ultimately leading to his tragic fate of murdering lookalikes.
When morning arrives, Claire enters the house to find signs of a struggle. After realizing the gun Owen used is missing, she hurries to the dock with Mel, where they discover Beth adrift on a boat with the firearm. In the realm of Nothing, the entity pressures Beth to commit suicide, but she boldly chooses life, setting the gun aside. Returning to reality, she finds Claire swimming toward her, eager to ensure her safety. Yet, as she arrives ashore, Beth catches sight of the entity's outline in the boat. When Mel asks what she sees, she answers, "There's nothing there," to which Beth knowingly replies, "I know."
