S2E7 · 3

In Los Angeles, Garrett Lorre, a middle-aged businessman, embarks on a one night stand with an anonymous woman he has met at a corporate party. As they are having sex in his hot tub, the woman bites Lorre to drink his blood. Two other men join the woman, helping her kill Lorre by repeatedly stabbing him using hypodermic needles.

The following day, before departing for L.A., Fox Mulder stores the missing Dana Scully's FBI badge in an X-file under her name. At the crime scene he meets with LAPD detectives investigating the case, explaining that Lorre's murder is the latest in a series of seemingly vampiric serial killings that have spanned two other states. Because the killers write biblical passages in the victims' blood, Mulder believes that they view themselves as an "Unholy Trinity."

Mulder visits a local blood bank where a night watchman has been recently hired. Mulder has him arrested after he is caught drinking blood in the facility's storeroom. During his interrogation, the suspect tells Mulder that he belongs to a trio of vampires who desire immortality; he is known as "The Son" while the other two, a man and a woman, are called "The Father" and "The Unholy Spirit". Mulder does not believe The Son's claims. However, at sunrise, The Son is burned to death when sunlight from the window touches his flesh. Mulder is taken aback, having previously assumed vampires to be purely mythological.

During an examination of The Son's body, Mulder discovers a stamp for Club Tepes, a local vampire club. There, he comes across a young woman named Kristen Kilar, who partakes in the consumption of blood. His suspicion aroused, Mulder follows Kristen after she and another club patron, David Yung, leave for an erotic liaison. He initially fears that Kristen is targeting Yung, but is beaten by Yung when he catches the agent spying on them. After Mulder leaves, Yung is murdered by the two killers, although the reason is not known.

Mulder runs a background check on Kristen, discovering that she formerly lived in Memphis and Portland —both the locations of earlier murders. Mulder assists the LAPD in searching her home, where he finds various blood-related paraphernalia. When she arrives later, Mulder is waiting for her. Kristen tells Mulder that she met The Son in Chicago and that they had engaged in "blood sports" together. Later, she fled The Son as he formed the Unholy Trinity with his accomplices and began their killing spree, following her across the country. Mulder and Kristen soon kiss while The Son, who has returned from the dead, looks on. The next morning, The Son confronts Kristen and tells her that by killing Mulder and drinking the blood of a "believer", she will become one of them.

Kristen approaches Mulder with a knife but instead stabs The Father, who is hiding in the bedroom. The Son attacks Mulder but is subdued. Mulder and Kristen try to escape using a car parked in the garage, but the Unholy Spirit jumps onto the car and attacks Mulder after smashing the windshield. Kristen jumps into the driver's seat and drives into her, impaling her on a wooden peg on the wall. Kristen tricks Mulder into running outside while she goes back into the house and pours gasoline around herself and The Son. Kristen lights a match, blowing up the house and taking her own life in order to kill the other vampires. Firefighters and police find four bodies in the ruins while Mulder stares at Scully's cross necklace.