Dr. Robert Wieder and his family return home from a banquet honoring him as the San Francisco Bay Area Doctor of the Year. Feeling joyous and celebratory, Dr. Wieder, his wife Nan and daughter Lucy prepare for bed, and invite the doctor's father-in-law, Dr. Irving Thalbro to stay the night. Dr. Thalbro is startled in his room by a haggard, rough-looking man standing behind him. The security alarm's motion sensor wakens the Wieders. Dr. Wieder goes downstairs to investigate, and finds Thalbro's body, hanging from the skylight, covered in blood. Scrawled in blood on the wall is the word "THEEF."

The agents are called to the crime scene, where Mulder ponders the word's meaning. There is no evidence that it is a case of murder. Mulder theorizes that the cause is hexcraft when he finds dirt splayed on the victim's bed, since dirt is commonly used to cast spells. Scully uncharacteristically agrees that this is a possibility. Mulder is surprised at her acquiescence.

Across town, in a run-down building, the landlady smells something suspicious in one of the apartments. She knocks on the door of its tenant, Mr. Peattie, who is the same man from the Wieder break-in. He tells her that he is cooking medicine and offers her a poultice for her sore back. She goes, leaving Peattie to finish sewing a number of crudely designed voodoo dolls. One of them hangs by the neck in the same manner as Dr. Thalbro.

Mulder finds out that the mysterious dirt was from a graveyard, meaning that it is conjure dust, a powerful tool for magick arts practitioners. Scully tells Mulder that Dr. Thalbro suffered from Kuru, a New Guinea disease that is extremely rare. Since Kuru causes irrational behavior, Scully concludes that Thalbro must have killed himself. Mulder believes that the disease was intentionally inflicted on the man so that he would kill himself.

Nan Wieder is disturbed when she discovers that a family photo was removed from the house. Dr. Wieder tries to console his wife, but when he pulls back the covers of their bed, dirt in the shape of a stick figure is on the sheets. Suddenly, Nan lapses into convulsions, and cyst-like sores erupt all over her body. As the doctor attends his wife, he doesn't notice Peattie standing outside the house, clutching one of his voodoo dolls with Nan's photo sewn inside.

At the hospital, Dr. Wieder tells Mulder and Scully that his wife has contracted a rare disease common only in central Africa. Angered by Mulder's folk magick theory, Wieder ignores Scully's offer of protection. The agents inquire at a magic shop about the use of conjure dust for murder. The proprietor tells them that in order to kill someone the magickian must draw power from a charm that holds great personal significance. The only way to prevent his power is to separate him from such a source.

Dr. Wieder studies his wife's MRI results and is shocked to see the word "THEEF" inscribed in the scan. Suddenly, Peattie appears behind him. "Truth hurts, don't it?" the man laughs. When the doctor asks why he is harming his family, Peattie utters "Lynette Peattie." Wieder does not recognize the name, and Peattie disappears. Confused, Dr. Wieder runs a record search, but no Lynette Peattie turns up. Then he looks for "Jane Does" under his care and finds one patient's file has a personal effect in it. It is a handwoven wristband, with the inscription "Flax hair lamb, theef of mans heart." This triggers Wieder's memory.

Later that night, Peattie buys a bag of popcorn from a hospital vending machine and cooks it in a machine he's never seen before -- a microwave. Meanwhile, Nan recovers, but must go through another MRI exam. As she slides into the radiology tube, Peattie removes the popcorn from the microwave and replaces it with a poppet doll. Inside the cylinder chamber, Nan begins to convulse and her body burns to a charred black. Peattie pulls the Nan doll out of the microwave and calmly says "All done."

When Dr. Wieder sees that the word "THEEF" was branded into his dead wife's chest, he tells Mulder and Scully about the strange man who confronted him. He also recounts the story of Lynette Peattie, a dying girl he had treated after a bus accident. Wieder had deliberately upped her morphine dosage to ease her pain even though the overdose would take the last twenty minutes of her life. Mulder realizes the killer must be Lynette's father, so he and Scully track down the girl's gravesite. They exhume the coffin, but it is empty.

Back at Peattie's apartment, the landlady lets herself in to look for another poultice. She gasps at Lynette's skeletal remains in the bed. She turns to leave and runs into Peattie.

The agents send Dr. Wieder and his daughter off to a safe house cabin, with Scully accompanying. Unseen, Peattie watches them go. The news reports a woman suffering from a sudden onset of a rare, flesh-eating disease, and Mulder quickly connects it to Peattie. When he reaches the apartment, Lynette's skull has been removed from the body. He phones Scully to warn her that their suspect may be heading towards the Wieders. Outside the cabin, Peattie waits. His daughter's head rests in a box on his lap.

Peattie breaks into Scully's car and takes her ID photo and a strand of hair from the seat. He puts them into a poppet and pokes the doll's eyes with coffin nails. Scully becomes blinded and her eyes are cataracted milky white. She stumbles about, and shoots blindly at Peattie when he bursts through the door. He merely knocks her gun aside, heading upstairs after the doctor and his daughter.

Dr. Wieder tells Peattie that he did everything medically possible for the man's daughter and that he eased her suffering when he could do no more. Peattie says that if he had been there, he could have saved her himself. He pulls out another cloth doll and twists a knife into it. Dr. Wieder falls to the floor, clutching his heart.

Mulder arrives at the cabin, finding the hex doll used to blind Scully. Scully's sight returns instantly, and she shoots Peattie to save Dr. Wieder.

The next day at the hospital, Scully admits that she would have followed the same procedure as Wieder had she been faced with Lynette's prognosis. However, she wonders whether Peattie really could have saved his daughter. Mulder is once again surprised and appreciative of Scully's admission of belief. Nearby, Peattie lies comatose on a hospital bed, dependent on the marvels of modern science and medicine.