Skinner is infected with a strange disease triggered by nanotechnology in his body.
Assistant Director Skinner is unwell and horribly discolored in hospital. Twenty-four hours earlier, Skinner loses a boxing match after experiencing a dizzy spell. He is discharged from the hospital but Mulder and Scully witness a "bruise" on his ribs growing. After trawling through security footage from the entrance to the J. Edgar Hoover FBI Building, Mulder and Scully recognize a physicist who brushed past Skinner that same morning. Mulder and Skinner travel to the physicist's house but find he is being held hostage. Mulder apprehends one of the kidnappers, Tunisian Man. He doesn't speak English but Mulder does a background check on him.
The background check leads Mulder to Senator Matheson, which results in a dead end. Scully discovers Skinner's blood sample and, after checking, she finds that Skinner's blood contains multiplying carbon. Meanwhile, Skinner ends up in hospital following a gunfight in the FBI parking garage. Mulder and Scully reunite at the hospital, where Mulder tells Scully that Skinner was investigating a health funding bill. The physicist dies from the same carbon blood condition that Skinner is sick from.
Skinner remembers having seen, on numerous occasions, a bearded man who showed up suspiciously and is actually running the scheme. He saves Skinner and sacrifices one of his own men. The case is closed and Skinner is, once again, mean to the agents, ordering them to report exclusively to Kersh. The bearded man was actually Krycek, who continues to control the potentially debilitating nanotechnology in Skinner's system.
- This episode features the return (and final appearance) of Mulder's source inside Congress, Senator Richard Matheson.
- In this episode we learn that Krycek has almost total control over Skinner and can blackmail him for his own means.
- The man that Mulder and Skinner encounter and pursue at Dr. Orgells home is identified as a Tunisian diplomat, with adequate credentials. His nation of origin is likely a reference to involvement by Conrad Strughold, the apparent leader of the Syndicate who is based in Tunisia.
- A potentially time-consuming and expensive fight scene between Skinner and Krycek was cut due to budgetary reasons.
- Nicholas Lea wasn't credited in the opening act so as not to ruin the surprise of his appearance.
- This is the fourth attempt to break Skinner away from Mulder and Scully. In 'Piper Maru' he was shot by Luis Cardinal, in both 'Zero Sum' and 'Avatar' they tried to frame him for murder, and here they infect him with nanites which Krycek eventually uses to blackmail him.
- Skinner: I woke up. Mulder: Alone? Skinner: Yes. Alone. This is a reference to the season 3 episode "Avatar" where Skinner has a one night stand with a women in a bar who turns up dead the next morning and Skinner is accused of her murder.
- Trainer: Don't telegraph that left, Drey. Stevie Wonder would see that one coming. Stevland Hardaway Judkins best known by his stage name Stevie Wonder is a famous singer who has been blind since just after birth.
- Although done in an X Files way, the main thrust of the plot is based on DOA (Dead on Arrival), a film originally made in 1950 with Edmund O'Brien, then remade less successfully in 1988 with Dennis Quaid. In both films, a man discovers he has been incurably poisoned and only has a short length of time to discover who has effectively murdered him. Sadly, as they didn't have Scully to try to find a cure, or Skinner's tormentor pulling the plug, they actually are DOA.
- Nurse: At least you didn't get your ear bitten off. A reference to the infamous Mike Tyson/Evander Holyfield fight, when Tyson bit off part of Holyfield's ear. Both the nurse and the doctor treating Skinner make reference to it.
- Title: S.R. 819 This episode may have been named for a "real" Senate Resolution (S. 819) from 1993 regarding a hazardous, toxic, and potentially deadly chemical: Trifluoromethylaniline. The bill extended the temporary suspension of duty on the chemical.
- When Skinner is almost shot in the parking garage (22:10), he fires two shots and gets out of the car. You can see the slide on his gun is back-as if he is out of ammo. When the camera pans back to him, the slide is forward. Additional comment: Right after Skinner collapses against the car in the parking lot, if you look closely, you can see the slide once again locked back on his pistol.
- During the final scene of the episode, where Skinner is seen conversing with Krycek within his car, the latter uses his left hand to shut down the device responsible for Skinner's illness. By this time on the series, Krycek only has his right arm, since the left was amputated. (TXF: "Terma")
- Raymond J. Barry as Senator Richard Matheson
- John Towey as Kenneth Orgel
- Kenneth Tigar as Dr. Plant
- Jenny Gago as Dr. Katrina Cabrera
- Donna Marie Moore as ICU Nurse
- Greta Fadness as OR Nurse
- Dan Klass as Forensic Tech
- Susana Mercedes as Driver
- Tim Van Pelt as Surgeon
- Keith Coulouris as Intern
- Arlene Pileggi as Arlene
- Al Faris as Silk Shirt Man
- Jonathan Fraser as Uniformed Cop
- Julie Hubert as Exam Room Nurse
- Mickey Knox as Trainer