Agent Reyes theorizes that a maniacal killer who critically wounded Doggett may have entered our world from a parallel dimension.
Working undercover, Reyes surveils Erwin Lukesh, a man the FBI suspects is a psychotic killer who cuts out his victim�s tongues.
Reyes follows Lukesh into the stairwell of an apartment building. Suddenly, Lukesh steps up behind her, a straight razor in hand. Doggett and Follmer, who had been monitoring Reyes via electronic surveillance equipment, hear a scream. They rush to Reyes, whose throat has been cut and is dying.
Doggett rushes into a nearby alleyway, where he orders Lukesh to stop. Doggett�s gaze leaves Lukesh temporarily, and when he looks back, Lukesh has disappeared, seemingly having vanished into thin air. Suddenly, Lukesh appears behind Doggett and aims Reyes� gun. He fires.
The story continues at Reyes� apartment, where Doggett has arrived with a housewarming gift: Polish sausage. Their conversation is interrupted by a phone call from Skinner, who tells Reyes that Doggett has been shot and is being rushed to a hospital. Reyes cannot fathom what Skinner is talking about, since Doggett is in her apartment. Or so she believes. When she turns to Doggett, he has disappeared.
Reyes makes her way to the hospital, where she tells Follmer her fantastic tale. When a ballistics test matches the bullet removed from Doggett�s body to Reyes� gun, Reyes is brought to a police station for questioning. During the interrogation, Reyes insists she was with Agent Doggett when Skinner informed her that Doggett had been shot. Unbeknownst to Reyes, Lukesh identified her as the shooter.
Scully and Skinner tell Reyes that Follmer�s case has some potential holes: Reyes was at her apartment at the time of the shooting, which is fourteen miles from the crime scene; her gun shows no evidence of fresh gunpowder residue.
A short time later, Doggett regains consciousness in his hospital bed, but he is critically wounded and breathes with the aid of a respirator.
Back at his apartment, Lukesh cares for his overbearing mother. While doing so, he opens the refrigerator and digs deep into the bacon drawer, eventually finding a freezer bag. Inside is a severed human tongue. He stares at it, transfixed.
Reyes performs a background check on Lukesh and grows convinced that he is responsible for the shooting. She returns to the hospital, where Doggett is able to communicate via a special computer device. She tells Reyes that she shot him. But he also states that he saw Lukesh cut her throat.
Meanwhile, Lukesh returns to the alleyway. He again vanishes into thin air.
When Reyes returns to the hospital for another visit, she develops a theory about what happened. She believes that the only thing that can explain events is the existence of a parallel universe... one that Lukesh can escape from at will. And since the Doggett she knows disappeared, she suspects it�s impossible for two Doggetts to exist simultaneously in the same world. In 'her' universe, Lukesh is a model citizen. But in the other universe, he�s a crazed killer.
Skinner and Follmer question Lukesh, who remains calm until his mother is mentioned. Sensing this, Follmer and Skinner press further. They hint of their intention to question Lukesh�s mother. When Lukesh returns home, he realizes that his mother discovered the revolver that he used to shoot Doggett. When his mother threatens to tell the FBI about her discovery, Lukesh kills her.
When Reyes returns to the hospital, Doggett tells her that the only way to put things right in both worlds is to shut off his respirator and let him die. Believing Reyes is Lukesh� next victim, Scully and Skinner monitor her movements in her apartment via hidden surveillance equipment. Suddenly, Lukesh grabs Reyes from behind and brings his straight razor to her jaw line. Follmer rushes in and opens fire, killing Lukesh.
Reyes then returns to the hospital, where she shuts off Doggett�s respirator. When Doggett dies, Reyes finds herself back at her apartment with Doggett, who is chewing on the last of his Polish sausage. Reyes is stunned. Tears in her eyes, she rests her head on his shoulder.