As a serial killer with an unusual method, Vann Siegert moves from one location to another, administering deadly doses of poison to his victims. He rationalizes his actions by arguing that he is merely helping them avoid suffering in death. After the death of Casper, a heroin addict he encountered at a bar, Vann cleverly manipulates the scene to mimic an overdose and relocates to a new town. The following day, he arrives at the home of Doug and Jane Durwin, who are grappling with their daughter's disappearance, and rents out her empty room. Doug suggests that Vann look for work at the post office, which is in need of temporary help for the holiday season.
As Vann begins to acclimate to his new life, he attends a high school football game with Doug, where he meets Gene, a star athlete, and his family. A short while later, Vann offers Gene a ride and cruelly murders him, hiding his body on a remote beach. While burying the remains, Vann engages in a mock conversation with detectives Blair and Graves, who question him about his practices in a mocking fashion. Subsequently, Vann participates in the community's efforts to locate the missing athlete and even attends his memorial service. The killing of Gene inadvertently violates two of Vann's personal rules: never kill someone you know and never harm anyone from his community.
Deepening his connections in the town, Vann earns more responsibility at the post office. One of his coworkers, Ferrin, exhibits an awkward attraction to him. Vann takes her for a drive to the beach, where they share a tense hug right above the site where Gene's body is concealed. On Christmas Day, Vann visits a diner and selects another target. She invites him to her house, revealing herself to be a painter. The unsettling nature of her artwork causes Vann to feel uneasy, prompting him to leave her home. Later, he returns to the diner and secretly poisons the drink of a solitary patron with a toxin derived from tree bark fungus found in the Pacific Northwest. An autopsy later links this rare poison to Casper's death once authorities discover his body. Vann understands that it is only a matter of time before the police establish a connection between him and the murders.
While looking in the mirror, Vann meticulously removes hairs from his jacket, placing them into an envelope marked “FERRiN,” signaling the soon-to-be consequences of his actions. As the investigation surrounding Jane's heinous murder progresses, Vann finds himself engulfed by paranoia, and his anxiety grows with the police's newfound interest in his status as a suspect. The tension escalates as he strives to maintain an appearance of normalcy during an unfortunate date with Ferrin, who is understandably rattled by his aggressive behavior. The evening concludes with his abrupt exit, leaving behind a cloud of discomfort and uncertainty.
The next day, Vann's darkest fears come to fruition when Doug is apprehended for Jane's murder, which leads the postal worker to hurriedly flee the town. Before leaving, he sends off his postal uniform and a mysterious envelope inscribed with “FERRiN,” containing a sample of his own hair, through the mail. As he distances himself from the turmoil, Vann depicts a longing for a new beginning, suggesting a desire for a more ordinary life.
Yet, his plan to escape is momentarily interrupted by a chance encounter with a sharp police officer, who had previously spoken to him on the beach. Their paths cross again as they travel down a desolate highway, the only sound being the quiet hum of Vann’s engine and the gentle rustle of leaves. As the officer takes the right fork in the road, Vann turns left, their parallel journeys serving as a poignant reminder that some secrets can never truly be left behind.
