Scully removes Mulder's nameplate from his desk, only to replace it when Doggett arrives -- marking the office as Mulder's territory despite his continued absence.

Having established the boundaries, Scully debriefs Doggett on what is to be their first case together. Two people -- George the mortician and his wife -- are dead of blood loss, their hands, torsos and heads covered with bites that appear human. Two of the man's fingers were gnawed off and are presumed eaten.

The agents have no choice but to travel to Burley, where Abbott, the detective in charge, suggests that the killer may not have been human. His evidence? A four-toed footprint near the bodies.

Doggett finds the victim's missing fingers in the attic. Based on the smell, Scully intuits that the fingers were regurgitated. The agents find another set of scratches -- claw marks? -- on a rafter.

Across town, old Mrs. McKeeson sits in her attic and looks through a photo album. The bat creature bursts from the shadows and attacks her.

Scully's forensic examination of the undertaker's body points to a non-human perpetrator. She finds traces of anti-coagulants unique to bat saliva in the wounds.

Doggett shows Scully a 1956 article about a series of mutilation murders in Montana performed by a human bat "with sharp teeth and four-toed claws." He concludes that the creature, which was never caught, "is back and killing again."

Shortly thereafter, the agents investigate the attack on Mrs. McKeeson. Scully suggests that the killing relates to the victim's daughter Ariel, who disappeared in 1956 and whose burned corpse was pulled out of the river the week before. Scully concludes that she is the connection to the earlier murders.

Since Scully is the local authority on paranormal phenomena despite her claims to be a simple scientist "who happens to have seen a lot," Doggett convinces Detective Abbott to follow her lead.

Regardless, Doggett complains that the intuitive leaps that the X-Files require run counter to his instincts and police training -- even in a case which appears to involve a long-lived human bat.

Elsewhere, a masked man enters a barn. He removes a bat from its perch, puts it in a small cage and carries it into his cabin. There is a dead bat nailed to the cabin door.

Abbott checks in with the workers at the cemetery. They tell him that someone dug up Ariel's casket before they arrived, pointing out the scratch marks on the coffin lid.

As the workers drive the body to the morgue, Abbott takes a look around the cemetery. The human bat bursts out of a hollow tree, leaps across the space separating them, and attacks.

Scully examines Abbott's corpse while the other cops on the force take Doggett to task for not capturing the creature earlier. They blame Scully for his death.

Despite the police animosity, Scully continues with her investigation. She tells Doggett that Ariel McKeeson's body was burned after she died in order to cover something up. Meanwhile, as she explains, all the recent victims had contact in one way or another with the body.

Only one person who had contact with the body -- Myron Stefanak, who pulled the corpse out of the river -- remains. Coincidentally, one of the hunters involved in tracking the Montana bat creature was named Stefanak: Ernie Stefanak.

They visit Myron Stefanak. Although Ernie was his brother, Myron insists that he disappeared around the time of the Montana murders. He refuses to discuss the case, and asks the agents to leave him alone.

Naturally, the agents stake out his house instead. While they wait, Scully worries that she is forcing the facts of the case to fit her predetermined conclusions. Doggett asks whether she thinks she's trying too hard to be Mulder.

We see the bat creature hanging from the barn rafters while Stefanak loads some supplies into his truck. Stefanak drives to the river and loads the supplies onto a small raft, using a pulley system to pull the raft to a small island in the middle of the river.

A masked man picks up the supplies under cover of darkness, only to find Scully and Doggett waiting for him. He is of course Ernie Stafanak, hiding on the island for the past 44 years.

What drove him to eschew society? As he puts it, "I know on the evolutionary ladder, bats are real close by the apes. And just as we came from the apes, so might a man have sprung from a bat. It would live and hunt like a bat, but with the cold-blooded vengeance of a man."

Ariel McKeeson was his wife. He arranged for her burned body to be found when she died so that she could be given a Catholic burial. The agents tell him that the creature has killed four people who had contact with the body, as though tracking Ernie's scent.

Ernie worries about Myron, but the agents say his brother was fine when they last saw him. He reminds them that bats only hunt at night.

Doggett leaves Scully with Ernie and goes to check on Myron.

The Bat Man attacks him on the shore of the island. While Doggett puts up a good fight, the creature is too powerful. Our new co-star slips beneath the water and doesn't come up again.

Ernie tells Scully that they shouldn't have come out to the island. He warns her, "The moment you stepped in here, you were marked."

The island's apparently advanced security system goes off to warn them that something is coming in out of the woods. This "something" lands on the roof, allowing Scully to not only brandish her gun but empty it into the roof, then go out to investigate.

While she's gone, the Bat Man comes in through the chimney and attacks Ernie. Scully runs back in and fires at the intruder, forcing it to leap past her into the woods.

Doggett arrives and both agents continue shooting at the fleeing monster. Later, as she tends to Doggett's wounds, Scully hears the creature's cries.

Back in Mulder's office, Doggett shows Scully a fax from Myron Stefanak, who has decided to go into hiding himself. Scully asks Doggett if he believes the creature will come back for them someday, but he insists that it must be dead.

Doggett mentions that their report on the incident is eliciting comment from "the guys upstairs." Scully tells him, "You get used to it."

Scully promises to get Doggett a desk of his own and thanks him for watching her back. Doggett replies, "I never saw it as an option. I'm sure you don't either."

As Doggett gets to work on the file, Scully puts Mulder's nameplate in a desk drawer.