"Medusa" is the twelfth episode of the eighth season of the American science fiction television series The X-Files.

The series centers on FBI special agents Dana Scully (Gillian Anderson) and her new partner John Doggett (Robert Patrick)—following the alien abduction of her former partner, Fox Mulder (David Duchovny)—who work on cases linked to the paranormal, called X-Files.

In this episode, a string of bizarre deaths in the tunnels of the Boston subway system sees Doggett join a team of professionals underground to investigate.

Later, when the train is back up and running, a group of commuters board the subway car and find the undercover cop with the flesh on half his face and his left arm stripped down to the bone.

However, they are rudely greeted by Deputy Chief Karras and Lieutenant Bianco of the transit police; the two eagerly want the FBI to get the job done fast so that the system can be reopened in time for the evening rush hour, and Karras is also irritated that Scully performed an autopsy on the body.

Even after an autopsy, Scully has no idea what killed the man, and the CDC is unable to find any biological or chemical agents in the subway.

The group includes Steven Melnick, a structural engineer, and Dr. Hellura Lyle, a CDC employee specializing in pathogens.

Scully, however, decides the plan will work better if Doggett acts as her eyes and ears while she analyzes the situation from up in the Transit Control Center.

When the group approaches the spot where the train lost power, Melnick starts crying out in pain.

Back in the tunnels, Doggett notices a green glow on Bianco in a dimly lit former subway station.

Doggett uses Bianco's gun as an electrical conductor from the third rail to the electrolytic water, killing the medusas and preventing further exposure.

She informs him that Bianco and Melnick are with plastic surgeons, the boy has been given to social services, and no criminal charges will be pressed against Karras since the electrical discharge from the third rail destroyed the proof of the medusas in the tunnel.

"[4] In real life, the Boston subway system is "fondly" called the T, short for the Massachusetts Bay Transportation Authority.

Second, the character Steven Melnick is named after the Vice President of Media Relations for 20th Century Fox Television.

"[10] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen gave the episode a mixed review and rated it two-and-a-half stars out of five.

"[12] Paula Vitaris from Cinefantastique gave the episode a slightly negative review and rated it one-and-a-half stars out of four.

[13] Despite noting, "after episodes about butt-crawling and vomiting, it's a relief to have one in which people die when their skin and muscles are fried away by microscopic sea creatures called medusas", Vitaris was critical of the plot, likening it to "cliche[d]" 1970's movies that featured the "obstreperous official insisting the plane must fly or the skyscraper is fire-proof, even when the potential for catastrophe is staring him in the face.

"[13] Spotnitz later admitted during a Reddit IAmA that the episode was his least-favorite that he had written because "the concept just wasn't clear or compelling enough to sustain the hour.