In this episode, offender profiler Frank Black (Lance Henriksen) investigates the initial outbreak of a deadly virus, and discovers that his employers, the Millennium Group may pose a danger to his safety.
"The Fourth Horseman" was written under the belief that the series would soon be cancelled, and inspired in part by the outbreak of bovine spongiform encephalopathy in the United Kingdom.
Twelve years later, Millennium Group consultant Frank Black (Lance Henriksen) is visited by a retired FBI agent, Richard Gilbert (Glenn Morshower).
Black meets with another Group member, Peter Watts (Terry O'Quinn), to investigate the death of Jason Mogilny, who was found at a riverbank surrounded by six pints of spilt blood, with no evidence of murder.
In El Cajon, California, a family sit down to dinner for Mother's Day before spontaneously collapsing and bleeding profusely.
At home, Black's wife Catherine (Megan Gallagher) tells him that Jordan has been having vivid nightmares about the apocalypse, in which she and her parents are isolated in a woodland cabin.
[3] The episode was the second of three to be directed by Dwight Little, who had previously helmed "Midnight of the Century",[2] and would return in the third season for "Borrowed Time".
Tiplady described her role in the episode as not being "just the cute Jordan Black with easy scenes and lots of giggling",[7] and added that the increased depth her character was given served to heighten her interest in acting.
VanDerWerff felt that the two-part finale was "at once haunting and terrifying", finding both "The Fourth Horseman" and "The Time Is Now" to be Millennium's most frightening instalments.
[12] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated "The Fourth Horseman" five stars out of five.