With Scully still missing, Mulder begins work on the newly reopened X-Files project. He investigates the violent death of a Los Angeles businessman who was attacked by several people as he used his jacuzzi. The murder resembles several other similar attacks believed to have been committed by the Trinity Killers: the victim's body is drained of blood, bite marks are discovered on the exterior jugular vein, and a quotation written in blood is found near the body.
     Mulder tracks his first suspect to the Hollywood Blood Bank. He finds a new employee in the basement dining on human blood. The man, who calls himself The Son, wants to live forever because he believes there is no afterlife. Mulder believes the man is delusional and a faker. Believing his condition a psychological, Mulder places The Son in a jail cell where he will come in contact with direct sunlight at dawn. Mulder tells him the sunlight will be blocked if he reveals the whereabouts of his accomplices. But as guards watch, horrified, The Son burns to death when dawn breaks.
     Mulder theorizes that vampirism is, in part, based on a real disease called porphyria, an affliction which causes blisters on the skin when a sufferer is exposed to sunlight. With the aid of a coroner, a hand stamp on The Son's skin is deciphered. It reads "Club Tepes." Mulder visits the club, where he meets a mysterious woman named Kristen. During an erotic encounter, Kristen tempts him with a drop of her blood. But Mulder declines to drink. An Asian man accepts Kristen's offer and the couple retreat to a secluded area. When Kristen finishes arousing the man, three of her accomplices rush him.
     Mulder and members of the police department discover the man's corpse drained of blood. Nearby is a can of cranberry sauce and some fingerprints. By matching the fingerprint with existing records, Mulder locates Kristen's home. Kristen recounts how she first met The Son, whose real name was John. During a fight with John, they tasted each other's blood. From that point on they turned to "blood sports." They were joined by others, but Kristen explains how it turned unnatural. Kristen insists that with John dead, she is not in jeopardy. She refuses Mulder's escort to the police department. Instead, Mulder spends the night with Kristen, and the two become lovers.
     The next day, The Son appears at Kristen's house as wild fires spread throughout the Malibu hills. The Son tells Kristen that he is immortal and cannot be killed even by sunlight. He instructs Kristen to kill Mulder. Instead, Kristen stabs The Father, the first of the Holy Trinity, who is hiding in the darkness. The Son attacks Mulder, but he is overpowered and tied up. As the fires near the house, Mulder and Kristen attempt to make their getaway by car. Suddenly, The Holy Spirit attacks the couple. Kristen hits the vampire with her car, impaling it against a nail. She returns to the house and pours gasoline around herself and The Son. She states her intent to become immortal and ignites the gas. Later, firemen tell Mulder that four corpses were discovered inside the house.