It was written by executive story editors Rob Des Hotel and Dean Batali and was directed by Bruce Seth Green.
The narrative follows Giles, whose friend has died, prompting the Scooby Gang to unravel his mysterious past; meanwhile, Buffy crashes into Ethan Rayne again.
As Giles is distracted by Buffy playing loud music in the library while doing calisthenics, he does not hear the man's cries for help at the door as the corpse strangles him, before falling to the ground and dissolving.
Shaken, Giles begins drinking in his grief and does not meet Buffy at the hospital, and she battles the doctor-dressed vampires alone until Angel shows up.
In the library, Buffy finds Giles' former friend Ethan Rayne, the costume shop owner who had caused chaos on Halloween.
Giles explains to Buffy that he ran with a bad crowd when he was young, and they used the demon Eyghon as a temporary high – directing him in and out of each other's bodies.
If you were to read basic loglines for ("Lie To Me") and "The Dark Age," the connection wouldn't be overly clear, and you'd presume that the episode is more or less just a continuation of the Ethan Rayne story from "Halloween."
This is corroborated in episode 16 of season 6 ("Hell's Bells") by Rory pointing out a poorly stuffed animal head at Xander and Anya's wedding.
It also provides insight into the character of Giles, who is central to the episode, and illustrates a common theme of Buffy, that "stains spread", that bad deeds can have long-running consequences.