Plot
At a psychiatric ward in Newark, New Jersey, therapist Rose Cotter meets a new patient, PhD student Laura Weaver, who recently witnessed her professor kill himself. Laura claims she is now being terrorized by an invisible entity that only she can see, that appears as various menacing smiling people, and has foretold her death. She begins screaming and choking, prompting Rose to call for help; Laura suddenly calms down and smiles, then fatally slits her own throat in front of a horrified Rose. That night, Rose sees a vision of a smiling Laura standing in the darkness of her kitchen.
The next day, Rose sees another patient named Carl, who smiles in the same way and shouts at her that she is going to die. Rose calls for nurses to restrain Carl, only to realize Carl has been asleep the entire time. Concerned for Rose's mental well-being, her supervisor, Morgan, orders Rose to take a week off. Rose's hallucinations rapidly increase in severity, and she visits her former therapist, Madeline, who suggests that Rose's problems stem from her childhood, in which she witnessed her abusive and mentally ill mother die from an overdose. Rose sees a toy train and buys it as a birthday present for Jackson, the son of her older sister Holly. At the party, he unwraps her present to find the corpse of Mustache, Rose's cat, horrifying everyone. Breaking down, Rose sees a guest smiling at her, causing her to fall into a glass coffee table and injure herself. This convinces Rose that she has fallen victim to a curse, although both Holly and her fiancé, Trevor, believe she has inherited her mother's mental health issues.
Upon learning that Laura's professor was smiling at her before his suicide, Rose visits the professor's widow, Victoria, and learns that he had also witnessed a suicide shortly before his. Rose asks her ex-boyfriend Joel, a police detective, to review old police records. They find several cases of people who witnessed someone commit suicide while smiling at them before doing the same to themselves within a week. Joel discovers the sole exception in the chain of suicides: convicted murderer Robert Talley. Rose and Joel visit him in prison, where he claims that the events are caused by an entity that feeds on trauma and that the only way to escape it is to brutally kill someone else in front of a witness to traumatize them, passing the curse to the witness, although Robert screams at them to leave when he realizes that Rose herself is cursed. While Rose contemplates what action to take, she is terrorized by the Entity when it disguises itself as Madeline coming over for a therapy session. Rose drives to her hospital with a knife and tries to murder Carl in front of Morgan, but this is revealed to be a hallucination. Rose wakes up in her car to find Morgan standing outside. He notices the knife, but she speeds away, prompting him to alert the police.
Rose drives to her abandoned family home, believing that the Entity's curse ends if she isolates herself so she cannot pass it on to anyone else. The Entity appears as Rose's mother, and it is revealed that Rose could have saved her mother but chose not to. The Entity attacks Rose, and Rose starts a fire during the struggle, seemingly killing the Entity. Rose flees the house and returns to Joel's apartment. When Joel suddenly smiles at Rose, she realizes this is another vision. In reality, Joel has tracked Rose's phone to her old house and found her outside the abandoned home. Realizing what is about to happen, Rose panics and runs back inside, where the Entity corners her and rips its skin off to reveal its true form: a skinless, semi-humanoid monstrosity with multiple sets of smiling jaws. The sight of the Entity's visage causes Rose to fall into a trance, and the Entity possesses her by pulling her mouth open and crawling inside. Joel breaks down the front door and witnesses Rose douse herself with fuel, turn to face him with a menacing smile, strike a match, and set herself on fire, passing the curse on to him.
