Michigan college student Jaime “Jay” Height sees a film with her new boyfriend, Hugh. In the theater, Hugh points out a girl whom Jay says she cannot see. Afraid, he asks that they leave. On another date, Hugh and Jay have sex in his car and he incapacitates her with chloroform. She wakes up tied to a wheelchair. Hugh explains that she will be pursued by an entity that only she can see, which can take the appearance of any person. Although it only moves at a walking pace, it will always know where she is and will be constantly approaching, and if it catches Jay, it will kill her and pursue the previous person to have passed it on: Hugh. After they see a naked woman walking toward them, Hugh drives Jay home and flees.
The next day, the police cannot find the woman or Hugh, who was living under a false identity. At school, Jay sees an old woman in a hospital gown walking towards her, invisible to others. Jay’s younger sister Kelly and her friends Paul and Yara agree to help and spend the night in the same house. Paul investigates a smashed kitchen window but sees no one; Jay sees a bloodied half-naked woman walking toward her. Jay runs upstairs to the others, who cannot see the entity. When a tall man with gouged-out eyes enters the bedroom, Jay flees the house.
With the help of their neighbor, Greg, the group discovers Hugh’s real name, Jeff Redmond, and trace him to his address. Jeff explains that the entity began pursuing him after a one-night stand, and that Jay can pass it to someone else in the same way. The group drives to Greg’s lake house, where Jay learns to fire a gun. The entity, taking multiple guises, attacks Jay on the lakefront. She shoots it but it recovers. Jay flees in Greg’s car but crashes into a cornfield, and wakes up in a hospital with a broken arm.
Greg sleeps with Jay, as he does not believe the entity exists. Days later, Jay sees the entity in the form of Greg. It then smashes the window to his house and enters. She tries to warn the real Greg on the telephone but he does not answer. She runs into the house and finds the entity in the form of Greg’s half-naked mother knocking on his door; it jumps on Greg and kills him. Jay flees by car and spends the night outdoors. On a beach, Jay sees three young men on a boat. She undresses and walks into the water. Back home, Jay refuses Paul’s offer of sex.
The group plans to kill the entity by luring it into a university swimming pool and plunging electrical devices into the water. Jay, waiting in the pool, spots the entity and realizes it has taken the appearance of her father as it throws the devices at her. Firing at an invisible target, Paul accidentally wounds Yara, but shoots the entity in the head, causing it to fall into the pool. As it pulls Jay’s foot underwater, Paul shoots it again and Jay escapes. Paul asks Jay if the entity is dead. Jay approaches the pool, which slowly fills with blood.
Jay and Paul have sex. Afterwards, Paul drives past prostitutes in a seedy part of town. Later, Jay and Paul walk down the street holding hands while someone walks behind them.