An undercover police officer is killed on a Boston subway, seemingly by a thug, though a good portion of his body has been eaten away by something. The transit cops are anxious to get the closed tunnel re-opened by day's end, though Scully wants it to remain shut down until they can find what killed him. Doggett and a police/CDC team head into the tunnel, and one is burned by some sort of toxic substance. Scully does a remote analysis of a nearby puddle sample and finds that it's only contains seawater.
The thug runs into Doggett as the team enters a decommissioned tunnel, and dies soon after. He and three other bodies in the tunnel all show the same signs of disintegrated flesh. Scully worries that some sort of contagion is at play, but revises her theory to a biochemical weapon when the burned cop is suddenly hit with a more electrical sort of attack on his arm. Doggett and a jumpy cop continue to pursue a shadowy person who has made a few appearances, and Doggett notices a luminescent material on the other man's face.
A marine biologist is called in and tells Scully that the sample contained a creature similar to one called a medusa. Doggett finds that he has also been infected with the luminescence, and Scully warns him that something unknown triggers it to the more unknown reaction. Meanwhile, both agents are annoyed that the transit chief is keeping with a deadline and allowing the evening trains to run. Doggett discovers that the mysterious person is a young, uninfected boy.
Scully thinks that the reaction might be triggered by sweat, and the boy is unaffected because his sweat glands aren't fully developed. He leads them through a leak which has brought the organism in and flooded the main lines. The trains start up again, and Doggett apparently manages to destroy the organism with an electrical charge transmitted from the third rail through a gun. Unfortunately, that leaves no evidence for Doggett's hope to punish the transit chief for recklessly endangering people.
Ken Jenkins, who is best known for his role as Dr. Bob Kelso on Scrubs, plays Deputy Chief Karras here. Penny Johnson, who plays Dr. Hellura Lyle, also played Sherry Palmer on 24 and Beverly Barnes on The Larry Sanders Show.
Episode Body Count
Officer Philbrick: killed by a reaction to the contaminated seawater.
Thug: also killed by the infected seawater.
Three squatters: found hidden in a decommissioned tunnel with the same lack of flesh.
Cumulative Body Count (173/202 episodes, 1/2 movies)
Humans: 1,913Creatures: 126
Grand Total: 2,102