After suspecting that their police officer neighbor is a serial killer, a group of teenage friends spend their summer spying on him and gathering evidence, but as they get closer to discovering the truth, things get dangerous.

In the summer of 1984, the small seaside town of Cape May, Oregon is plagued by the "Cape May Slayer", a serial killer responsible for the disappearances of 13 teenage boys in the county over the decade prior.

Fifteen-year-old Davey Armstrong (Graham Verchere), the son of a journalist who breaks the story, suspects that his neighbor Wayne Mackey (Rich Sommer), a popular police officer in their hometown of Ipswich, is the Cape May Slayer. Davey's friends whom are Dale "Woody" Woodworth (Caleb Emery), Curtis Farraday (Cory Grüter-Andrew), and Tommy "Eats" Eaton (Judah Lewis) reject this theory, on account of Davey's reputation for conspiracy theories and urban legends.

However, when a boy Davey sees inside Mackey's house appears on the back of a milk carton days later, they agree to help him investigate.

The four teen boys document Mackey's daily routine and discover many suspicious activities; Mackey regularly purchases gardening tools and bags of soil, brings a duffel bag to work every day, and goes on routine late-night jogs. One night, when the friends stake out Mackey's house under the guise of a game of manhunt, Mackey witnesses Davey plant a walkie-talkie outside his window, leaving Davey worried that he is becoming suspicious.

Later, Woody and Farraday discover a second vehicle and canisters of sodium hydroxide in a self-storage room owned by Mackey, and Davey and Eats discover the bloodstained shirt of the missing boy, hidden in Mackey's garden shed.

They present their evidence to Davey's parents, who are outraged at the boys, calling their investigation vandalism. Mr. Armstrong brings the boys to Mackey's house and has them apologize. Mackey, who claims that the boy seen inside his home was his nephew, expresses no hard feelings, but Davey is grounded.

Davey later sneaks out with Nikki Kaszuba (Tiera Skovbye), his friend and the boys' mutual crush, who insists that Davey abandon the investigation. The next day, Mackey visits Davey's home and attempts to call his nephew as proof of his innocence, but the call is not answered. Davey discovers that Mackey dialed his own home phone number, and relaunches the investigation.

The following day, a suspect is arrested in the Cape May Slayer case, with Mackey the arresting officer. Disgusted, Davey makes plans to break into Mackey's home during the 1984 Cape May Festival. Farraday, who attends the festival as a lookout, discovers that the bags of soil were purchased for a city beautification project, and he and Eats abandon their posts.

Davey, Woody and Nikki enter Mackey's home with Mr. Armstrong's video camera and explore a locked room in the basement, apparently decorated to resemble Mackey's childhood room. They enter an adjoining bathroom and are horrified to find the missing boy's desiccated corpse in the bathtub, along with a still-living recent abductee. Mackey really is the Cape May Slayer all along. They present their footage to the Ipswich Police Department, who issue an APB on Mackey.

That night, Mackey abducts Davey and Woody, and abandons them in his cruiser on an offshore island, announcing that they are to play a game of manhunt. The boys flee into the wilderness as Mackey pursues, but lose their footing on a corpse dump. Davey distracts Mackey long enough for Woody to make a break for the cruiser, but Mackey overpowers Woody and slits his throat. He corners Davey, poised to kill, but decides instead to spare him - to leave him paranoid and constantly expectant for Mackey's eventual return.

Rescued and returned to a normal life, Davey retraces his morning paperboy route, passing the abandoned home of a missing neighborhood boy; Eats's house, trashed by his fighting parents; and Mackey's house, plastered with police tape. He unfurls a newspaper, the headline announcing that the Cape May Slayer is still at large.