The show centers on FBI special agents John Doggett (Robert Patrick) and Dana Scully (Gillian Anderson)—as well as ex-FBI agent Fox Mulder (David Duchovny)—who work on cases linked to the paranormal, called X-Files.
In this episode, Mulder, Walter Skinner (Mitch Pileggi), and Doggett come up against the horrible consequences of the Syndicate's pact with the aliens, as Billy Miles (Zachary Ansley)—now reprogrammed as a Supersoldier—attempts to erase all evidence of the tests—including Scully's soon-to-be-born baby.
Upon hearing information from Alex Krycek (Nicholas Lea), the men reluctantly call upon him as well as Monica Reyes (Annabeth Gish) to help them.
In addition, Nicholas Lea reprises his role as Alex Krycek, who temporarily switches sides and aids Mulder and Scully.
Meanwhile, Billy Miles (Zachary Aynsley), satisfied with the research conducted at Zeus Genetics, kills the head scientist, Dr. Lev, and burns the laboratory.
Fox Mulder (David Duchovny) informs John Doggett (Robert Patrick) of the fire, and asks to go along to survey the crime scene.
As they enter Parenti's office, they confront Miles—Mulder is thrown through a glass barrier while Doggett shoots him several times with no apparent effect.
As Doggett, Mulder, and Walter Skinner (Mitch Pileggi) survey Haskell's murder scene, Scully catches Gill in the act of tampering with her medication.
[3] The episode, which was the first part of two and concluded with the season finale "Existence", was written during a time of uncertainty for the series.
As such, portraying actor Lea explains the character's motivation: "Toward the end, he realizes that it's possible that the world could completely go down the tubes—then he's got a stake in trying to keep that from happening.
[7] In the United Kingdom and Ireland, the episode made its first televised appearance on June 28, 2001, on Sky1 and was the third most watched program that week, receiving 0.65 million viewers.
Handlen was complimentary towards the installment's "tremendous sense of purpose [that] builds to a fever pitch that makes questions of who exactly wants what largely irrelevant.
"[13] While he offered a pessimistic opinion in regards to the series' mythology as a whole, Handlen concluded that the eighth season's sense of purpose, as exemplified in "Essence", made the episode successful.
Wigmore gave the episodes a 9 out of 10 rating and wrote "the reason that this two-parter works is that its plot is simple enough for the audience to still have a handle on".
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 more mixed two stars out of five.
[17] The two cited dense dialogue and bizarre characterization—most notably the fact that "there's usually one FBI agent so angry that he interrupts [...] only to be stopped by a calmer counterpart"—as detractors.
[18] She heavily derided the plot, noting that the episode turned Scully into a "passive, mostly silent Virgin Mary who is about to give birth to… Jesus Christ.