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Mulder investigates a trio of serial killers who call themselves "the Father", "the Son" and "the Unholy Spirit", and who may be vampires.

Tropes:

• Arbitrary Skepticism: After one of the vampires burns alive on exposure to sunlight, Mulder — who had previously shown no trouble believing in the likes of aliens, superpowered mutants and poltergeists — sheepishly admits that he had rejected the existence of vampires out of hand. (Of course, next time he meets them, he knows better.)
• Hemo Erotic: The sexy vampire Kristen Kilar, who licks her own blood and blood of other people. She has some sexually explicit scenes, including the one where she is shaving shirtless Mulder.
• Not Quite Dead: The man who burns up in the police station comes back as a vampire.
• Our Vampires Are Different: In-Universe too, as they differ from the vampires we see in "Bad Blood".
• Perp Sweating: Mulder refuses to cover the windows unless the suspect talks. Interestingly in this case he doesn't believe that the man is a vampire (he can be seen in a mirror despite claiming otherwise), just that he's delusional and the sunlight will give him an excuse to confess.
• Red Herring: We're initially led to believe that Kristen is the "Unholy Spirit", the female member of the vampire trio, but it turns out that she actually left them years ago and, in fact, was never fully turned; she needs to taste the blood of a believer and take a life first. She still has the need to consume blood, but lacks the vulnerability to sunlight or (it's implied) the immortality. It also means that if she kills one of them, they just come back to life, since they can only be permanently put down by one of their own. However, she solves this by tasting her own blood and taking her own life, blowing herself up along with them.
• The Scully: Mulder flirts with this trope; he doesn't believe he's dealing with vampires until a subject spontaneously combusts in the sunlight.
• Symbolic Blood: Happens frequently throughout the episode, with the camera lingering meaningfully on puddles of things like red wine and raspberry sauce.
• Tragic Keepsake: Mulder now wears Scully's gold cross.
• Vampires Own Night Clubs: Okay, the vampires don't seem to own the nightclub, but it does seem vampire-themed ("Club Tepes"?) and they hang out there.
• Who Wants to Live Forever?: Averted; the Vampire Wannabe says the price he'll pay for immortality won't seem too high when Mulder is an old man while he'll be as young as he is now.
• Written-In Absence: Scully, because of Gillian Anderson's pregnancy, for this episode.