"Tooms" is the twenty-first episode of the first season of the American science fiction television series The X-Files, premiering on the Fox network on April 22, 1994. It was written by Glen Morgan and James Wong, and directed by David Nutter. The episode featured Mitch Pileggi's first appearance as Assistant Director Walter Skinner and saw Doug Hutchison and William B. Davis reprise their roles as Eugene Victor Tooms and the Cigarette Smoking Man, respectively. "Tooms" received positive reviews from critics, and it earned a Nielsen household rating of 8.6, being watched by 8.1 million households in its initial broadcast.

The show centers on FBI agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files. When mutant serial killer Eugene Tooms, last seen in "Squeeze", is released from prison, Mulder and Scully try to stop him from resuming his killing spree. Tooms, however, sets out to frame Mulder for assault before the agent can apprehend him.

After seeing men working on an open escalator in a mall around Christmas time, Morgan decided to revisit the character of Tooms. "Tooms" introduced the character of Walter Skinner, although this would be his only appearance in the first season. That character was conceived as playing against the stereotypical bureaucratic "paper-pusher", being instead someone more "quietly dynamic".

After the events of "Squeeze", Eugene Victor Tooms has been placed in a Baltimore sanatorium. He attempts to escape by squeezing his arm through the food slot of his cell door. He is forced to abort when he is visited by his psychologist, Dr. Aaron Monte, who tells him that he is ready to be released into society.

Dana Scully is called before FBI Assistant Director Walter Skinner, who is accompanied by The Smoking Man. Despite the success of the X-Files investigations, Skinner criticizes their unconventionalism and wants both Scully and Fox Mulder to do by-the-book work. Later, the agents attend a release hearing for Tooms, where Monte claims that Tooms' attack on Scully was due to being falsely accused of murder. Mulder tries to point out the evidence of Tooms' crimes and unusual physiology but is ignored by the hearing's panel. Tooms is released into the care of an elderly couple and is ordered to continue his treatment with Dr. Monte.

Scully meets with Frank Briggs, the retired detective who investigated Tooms' 1933 murders. Briggs claims that the body of one of the victims from that spree was never discovered. He and Scully visit a chemical plant where a piece of the victim's liver was found, ultimately discovering a skeleton encased in concrete. A researcher examining the skeleton identifies it as the missing victim from 1933. However, there seems to be no substantial evidence proving that Tooms was the murderer.

Meanwhile, Mulder harasses Tooms at work as he stalks a would-be victim. Later that night, Mulder follows him when he tries to break into a couple's house. Tooms flees without attacking anyone. Scully relieves Mulder, who is watching Tooms' new residence; they are unaware that Tooms is hiding in the trunk of Mulder's car. He manages to break into Mulder's apartment, where he injures himself and imprints Mulder's shoe print on his face. Tooms' frameup leads the police to question Mulder and Skinner to forbid him from contacting Tooms.

Further research on the skeleton reveals bite marks matching Tooms' teeth. When the old couple watching Tooms leave their home, he is visited by Monte. Tooms kills him and consumes the final liver he needs to begin his thirty-year hibernation. After discovering Monte's body and defying Skinner's orders, Mulder and Scully head to the site of Tooms' former apartment building, which has been demolished and replaced with a shopping mall. Inside, Mulder crawls below an escalator and finds Tooms' nest. Tooms bursts out, covered in bile, and pursues Mulder, who makes it to the surface and activates the escalator, trapping and killing Tooms.