With Scully still missing, Mulder begins work on the newly-reopened X-files. 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 a group called the Trinity Killers. They are known to leave a victim's body drained of blood, bite marks on the exterior jugular vein, and a quotation written in blood 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 thinks the man is a delusional fraud, and, believing his condition psychological, places The Son in a jail cell where he will come in contact with direct sunlight at dawn. Mulder tells him the sun will be blocked if he reveals the whereabouts of his accomplices. 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 nightclub and meets a mysterious woman named Kristen. During their encounter, Kristen tempts him with a drop of her blood. Mulder declines to drink, citing his fear of contracting a disease. An Asian man accepts Kristen's offer, and the couple retreat to a secluded area. When Kristen finishes entertaining 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, but instead, spends the night with him.

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, but 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 impales the vampire with her car, and she returns to the house to pour gasoline around herself and The Son. Because only a vampire can kill another vampire, she tastes her own blood, the blood of a believer, to become a vampire herself. She sets the place on fire.