Annie Marshall bursts out of her home, seemingly on the run, yet she insists she doesn't need assistance from those nearby. She hops into her car and speeds away. That evening, she finds herself alone on a beach, where she makes a heartfelt call to her parents, expressing her love for them. Tragically, the next morning reveals her brutalized body lying on the sand.
Meanwhile, carefree university student Jay Height attends a movie with her boyfriend, Hugh. During the film, Hugh's attention is drawn to a girl dressed in yellow, but Jay insists she can't see her. Feeling uneasy, Hugh suggests leaving. Later, in his car, Hugh and Jay have their first sexual encounter, but things quickly take a dark turn when Hugh uses chloroform to incapacitate her. When Jay regains consciousness, she finds herself bound to a wheelchair. Hugh reveals shocking news: he has transmitted a sinister curse to her through their intimacy—an invisible entity will now pursue her, one that they alone can see.
This entity moves at a slow pace, but it has an uncanny ability to know Jay's location at all times. If it manages to catch up to her, Jay's life will be forfeit and it will then go after the one who passed it to her. To illustrate the reality of the threat, Hugh waits until a naked woman slowly approaches them, urging Jay to act quickly and sleep with someone else to pass on the curse. He then drives her home, disappearing into the night. The following day, the police find both the mysterious woman and Hugh—whose real name is revealed to be Jeff Redmond—missing.
Back at school, Jay is unsettled when she spots an old woman approaching her, a figure invisible to everyone else around. That night, Jay's sister Kelly, along with their friends Paul and Yara, gathers at her home. Suddenly, someone throws a rock through a window; Paul looks around but finds nothing. Later, Jay is alarmed to see a ragged, half-clothed woman coming toward her and rushes upstairs to alert her friends, who can't see the danger. Jay's fear escalates when an unsettling tall man enters her bedroom, prompting her to escape on a bicycle.
With help from their neighbor Greg, the group uncovers the truth about Hugh's identity. Jeff explains that he was marked by the entity after a one-night stand and emphasizes the need for Jay to pass it on to someone else for her safety. He advises her to drive away to give herself time to formulate a plan. Greg takes Jay, Kelly, Yara, and Paul to his family’s lake house for some respite. The next day, while they enjoy the lakefront, the entity makes its presence known when it takes on Yara’s form and attacks Jay from behind. Fleeing in Greg's car, she crashes and wakes up in a hospital with an arm injury. In a desperate move to gain some time from the entity, Jay ends up sleeping with Greg in the hospital.
Despite Jay's insistence on the entity’s existence, Greg dismisses her fears. Soon after, Jay spots the entity disguised as Greg and panics, rushing into his house where she unexpectedly encounters the entity taking on the grotesque appearance of Greg's mother, leading to a tragic outcome. After that harrowing event, Jay finds herself isolated outdoors, contemplating her situation.
On the beach, she spots three young men on a boat, and in a moment of impulsive relaxation, she wades into the water. Back at home, Paul, willing to confront the entity’s threat, proposes that Jay pass it on to him. She declines, choosing self-preservation over passing the burden.
The friends coordinate one last attempt to defeat the entity by luring it into a swimming pool, intending to electrocute it. While waiting in the pool, Jay faces a horrific apparition when the entity appears as her father. Instead of entering the water, it hurls electrical devices at her, prompting chaos as Paul tries to defend her by firing at the invisible threat. In the struggle, Paul accidentally injures Yara but manages to shoot the entity multiple times, causing it to sink into the pool. Jay narrowly escapes as it pulls her down, only to find herself watching in horror as the pool fills with blood.
Later, back at Jay's house, she shares a moment of vulnerability with Paul, leading to intimacy between them. After a drive through town, where they pass sex workers, Yara recovers in the hospital. As the narrative unfolds, Jay and Paul walk down the street, hand in hand, only to have a figure lurking ominously behind them, hinting that the nightmare may not yet be over.
