S6E15 · Arcadia

At the Falls of Arcadia, a fictional planned community in San Diego County, California, disgruntled homeowner Dave Kline arrives at home to find a package from an unknown person. The package contains a tacky whirligig, which Kline puts on his roof to annoy the neighbors. While in bed that night, Kline hears an intruder in the house. He goes to investigate while his wife, Nancy, stays in bed. A mysterious creature attacks and kills the Klines.

Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) investigate the Klines' disappearance, going undercover as new homeowners under the aliases Rob and Laura Petrie. As Mulder and Scully are moving into the Klines' former home, neighbor Win Shroeder nervously tucks away Mulder's basketball hoop into the garage, telling him that it is against the community's CC&Rs. Settling in, the agents begin searching the house and find what appears to be blood on a blade of the ceiling fan. When Big Mike, another neighbor, wants to let the "Petries" in on the "consequences" of breaking the CC&Rs, homeowner association president Gene Gogolak describes him as "a weak link" to be dealt with.

That night, Big Mike disappears after being attacked by the creature. While taking a walk, Scully later finds the Schroeders' dog and Big Mike's necklace in a storm drain. The dog's face is covered in a substance that looks like blood.

Mulder and Scully discuss possible motives for the Klines' presumed murders, and Scully decides to have the substances analyzed in San Diego. Mulder decides to test his theories that noncompliance with the CC&Rs is the motive by sticking a plastic flamingo in the yard and damaging the mailbox, among other antics. Mulder later finds a note in the mailbox that says, "Be like the others... before it gets dark." After dark, Mulder brings out his basketball hoop, and Schroeder runs over to frantically argue with Mulder to put it away. Meanwhile, something comes out of the grass at Mrs. Schroeder, who screams. Mulder chases it away, but they all notice their light has burned out.

Schroeder confronts Gogolak, accusing him of trying to kill his family. Instead, Schroeder is told "Rob Petrie" is the real problem. Meanwhile, Mulder believes the creature that kills the homeowners moves through the yard, under the grass. Scully shares her lab results: the "blood" on the ceiling fan and on the dog is actually grime, as the neighborhood is built on top of an old landfill. Mulder believes the Klines were buried in their front yard, so the next day he gets a backhoe to dig it up, telling the neighbors he's putting in a reflecting pool, which is not against the CC&Rs. They don't find the Klines, but they do find the tacky whirligig that had been mysteriously delivered to the Klines before they died. The whirligig bears a label from Gogolak's company.

As Scully calls for a forensic team, she hears something in the house. She goes for her gun in the dresser drawer but finds it missing. As the creature comes up the stairs, a bloodied Big Mike grabs Scully and tells her to get out, saying that "it's coming" for her. He shoves Scully in the closet, and fights with the creature. Meanwhile, Mulder confronts Gogolak about marking the Klines for death by giving them the tacky whirligig. He realizes that the creature is a tulpa, a Tibetan thoughtform, that Gogolak conjured to assure compliance with the CC&Rs.

Mulder arrests him, handcuffs him to a mailbox and goes to find Scully. Cuffed to the mailbox, Gogolak begs for help, knowing the creature is coming. The creature attacks Gogolak and, as he dies, disintegrates into dirt. Scully comes outside too late to see the creature, the remnants of which are at Mulder's feet.