Lassiter is having a drink after work at a bar when he meets a woman named Marlowe, whom he is instantly attracted to. They hit it off immediately. After Lassiter tells her he is a cop, she asks to be excused to use the restroom. Several minutes pass by, and Lassiter thinks he has been stood up. Several minutes pass, and Lassiter checks the ladies restroom to see if Marlowe is okay. He doesn't find her. She has disappeared, and Lassiter is devastated.
The next morning, Lassiter arrives at a crime scene where a dead body was found. He gives special orders to McNab to find his mystery woman, Marlowe. They go to examine the body, and Shawn SV's puncture wounds on the man's jugular and wrist. Then Lassiter is devastated by the next clue he sees on the body: The same exact Eternal Life Symbol Necklace Marlowe was wearing at the bar. The woman he is infatuated with could be the person that murdered this guy. Upon seeing the puncture wounds on the victims wrist and neck, Shawn makes a bold proclamation that the killer was a vampire. And in a stunning turn of events, Lassiter jumps on board with Shawn's crazy theory and boldly agrees.
At the coroner Woody's office, Shawn, Gus, Lassiter and Juliet are told the body was drained of almost all its blood, thus causing the guys to further believe in their vampire theory. Juliet is the only person who isn't buying it.
Lassiter breaks off from Juliet. He heads to the Jewelry store where the Eternal Life necklace was likely purchased and questions the store clerk. The store clerk says those necklaces are very popular and does not have a record of who would have purchased it. The clerk admires Lassiter's watch, causing Lassiter to take a closer look at it. In doing so, he sees a single fingerprint - Marlowe's fingerprint!
Juliet, Shawn and Gus head to a vampire bar to get the skinny on who would be needing large amounts of blood. They meet a bartender who tells them that he got a strange call from someone asking where they could get their hands on a pint of blood. They trace the number and are lead to an address...
Meanwhile, Lassiter has found Marlowe and has successfully gotten her alibi for the night of the murder. Also, Lassiter sees that she is wearing her Eternal Life necklace, letting him know that it wasn't the same necklace that was found in the victim's hand.
Shawn, Juliet and Gus arrive at the house. They break in and find Lassiter there with Marlowe. They are shocked to see this, but Lassiter quickly tells them that she is innocent. Upon further investigation of the apartment, they learn that Marlowe has three male roommates - one of which is named LUCIEN. They question Lucien. It turns out he's clean. He explains that he was working at a mini golf place at the time of the murder. Shawn SV's a few things from the apartment... a cat litter box, a cracked mirror and an address. Lucien tells them that those things are all in his roommate ED'S room. They have the wrong roommate! With that, they head straight to the address Shawn SV'd. They arrive and see that they are at a blood bank.
At the blood bank, Shawn, Gus, and Juliet witness a series of strange occurrences that only further support Shawn's vampire theory. Then they see a cloaked figure break into the blood bank and flee the scene. They are unable to capture the suspect, but Shawn recognizes a jewel encrusted nail in the debris of the break-in and immediately makes the connection that Marlowe had the same exact fingernails. She must have been behind the break-in! They must get to Lassiter before she sucks him dry!
They arrive at Lassiter's apartment, and sure enough, Marlowe is there with him. Shawn presents the fingernail evidence, and Lassiter has no choice but to arrest her.
Later, at Marlowe's house, they find even more damning evidence against her: bags of blood she has stored and hidden. However, Marlowe's name is cleared from the murder when they get a call that another body has been drained of blood, but hasn't been declared dead yet. There's no way she could have done it because she was in police custody.
They head to the hospital where Shawn makes the connection that all the victims have the same blood type - O negative, an extremely rare type - and that only about a dozen people in Santa Barbara have it. Shawn discovers a name on the O negative blood list that tells him that ADRIAN VICCELLIO has the same last name as Marlowe, and that they must be brother and sister. Juliet then realizes a frightening piece of information... Lassiter has O negative blood type and therefore Adrian must be coming for his blood next.
Marlowe tries to warn Lassiter over the phone, but it's too late. Adrian lunges at Lassiter and covers his mouth with a rag drenched in chloroform. As Adrian sets up the equipment to drain Lassiter's blood, Lassiter comes to and Adrian tells him that he was a mark at the bar - that Marlowe was supposed to drug and drain him of his blood, but she didn't because she liked him. He manages to take down Adrian by himself just as Shawn, Juliet and Gus show up to save him. Lassiter tells the group that he was able to take down Adrian because he has been building up a resistance to chloroform for the last 15 years. And with that, Adrian is taken into custody.
Later, Lassiter finds himself completely distraught about Marlowe being in jail. He visits her and tells her that despite her criminal activity, he knows that she was just trying to help her brother, and tells her that he will wait for her until she gets out.