Scully is stranded in deepest Utah, where the townsfolk have an unusual attitude to hitchhikers...
Hank Gulatarski tries to hitch a ride on a bus in the Sevier Desert in Juab County, Utah. He's woken up when the passengers walk off in to the desert and beat a young, severely arthritic man to death with rocks. Then they turn on him.
Scully is called in to investigate by the local coroner. Her mobile has no signal, so she rings Doggett on a payphone to let him know where she is - he's annoyed she didn't ask him along, but agrees to research a file on a mucus-related murder.
Scully sets off in pursuit of a mysterious bus, but stops for petrol in a tiny town after losing it. The pump attendant fills her tank but up the road her car breaks down because the attendant has filled the car with water. Unable to phone from the petrol station, she tries Mr Milsap's boarding house - but his phone is dead. The only other entertainment is a group of people reading the Bible in some nearby shacks.
At the boarding house Milsap asks Scully to examine Hank - he has a wound on his spine, and a weird slug-creature under the skin on his back which Scully can't remove. Hank tells her about the killing of the arthritic man, and passes out. When he comes round, Scully tells him he's in danger, and could die - she goes looking for an escape vehicle. Hank tells Milsap and the bus driver that he's dying - they treat him with great reverence when he says 'another swap' may be required.
Back at base, Doggett is concerned that Scully missed her appointment with the Coroner and calls in Ciolino, the local sheriff. They examine pictures of murder victims with the same wound on their spines.
Scully finds the bus, but is attacked by the townspeople, who then crack open Hank's skull. taking out the slug, they hold it by Scully's back. Bound and helpless in the boarding house, she realises the slug is creeping towards her brain. The townspeople think she'll make a very suitable host...
Suspicious, Doggett arrives in town, calls for reinforcements and rescues Scully from the boarding house. They commandeer the bus, and Scully gets Doggett to cut out the slug with his switchblade. As the townspeople start attacking the bus, Doggett tosses away the slug and shoots it.
The town cult, it seems, believed the slug was the Second Coming of Christ. Doggett reprimands Scully for not inviting him along - she promises not to go solo again.