"Thirst" is the only Halloween-themed episode of the TV show Smallville. It first aired in the United States on the WB Television Network on October 27, 2005. Although the series always broadcast in October, it typically ignored the holiday, making this lone exception during its fifth year. The episode has Clark and Chloe actively investigating apparent vampires on a college campus. Its WB trailer was unusual for the series — highly stylised to hype the episode's Halloween-themed content, while wholly ignoring the rest of the episode's content.
It is also an important episode to the overall development of the show, as the story Chloe wrote about the vampiric sorority resulted in her landing a job at the Daily Planet — an important location for the remainder of the series.
Nevertheless, its contribution to the ongoing mythos of the show was not enough to make it a particularly popular episode with fans. As a statiscally invalid poll at tv.com reveals, it was one of the worst-received episodes of the series. Users of that site felt, as of October 2013, that it was the nadir of the first five seasons. "Thirst" was particularly poor for the first thirteen episodes of season five, which tv.com users rated as otherwise one of the most consistently strong runs of episodes in series history. Out of the show's entire catalogue, it was better than only three episodes, all of which came from what tv.com voters viewed as the clearly disappointing eighth season.
Chloe wants a job at the Daily Planet, but is turned down by the new editor-in-chief, Pauline Kahn. Although she does not get the job, she is emboldened to find the story that will get Kahn to give her a second chance.
Soon thereafter, she thinks she has found something weird going on at the Tri-Psi sorority. At the sorority house, a pizza boy arrives and the girls invite him into the hot tub. During the make-out session, the girls suddenly start biting him, he begins to scream and the water turns blood red.
Clark goes to visit Lana in Smallville, only to find her packing. She tells him that she has applied for late acceptance to Met U, but there is no longer any housing available, so she is considering a sorority. Clark is disappointed and suggests that she go to CKU with him, but she is determined to Metropolis. She assures him that nothing will change between them.
Later, the Tri-Psi sorority head, Buffy Sanders, introduces her group as the prettiest and most exclusive sorority. Lana is one of the pledges and is surprised to find out that she is the only one that has been picked to join. They tell her that there is one more initiation ritual, and it is revealed that the entire sorority are vampires. They bite Lana and she becomes part of the sisterhood. She eventually wanders off to Chloe's dorm room to recover from the biting.
Later, Chloe's roommate Karen is dismantling her mini version of the Wall of Weird when she arrives home. She is angry about the wall, and even angrier that Lana Lang is asleep in Chloe's bed. Karen storms away and Chloe tries to revive Lana. She is groggy and lethargic, and Chloe thinks she is hungover from the pledge party. Clark arrives and Chloe leaves Lana in his care. He asks if she iss feeling all right and she says she iss fine and starts making out with him. Clark is surprised and confused by her aggressive advances and she gets annoyed by his reluctance and leaves.
Chloe's investigation continues and she reports that Lana and her new friends have fun throwing each other off the house's balcony and getting drunk. Clark is worried about Lana's strange behavior and Chloe tells him that she has been investigating the sorority and has uncovered some strange details. None of the sisters ever leave the sorority, and it is the most exclusive one on campus. She makes a plan to infiltrate their Halloween party and brings Clark along. They split up to locate Lana. Clark ducks into a bedroom where Buffy catches him rummaging. She orders him to leave, but she is distracted by someone at her door. When she turns back to Clark, he is gone.
Chloe finds Lana dancing and making out with other boys and pulls her to the side. She asks her about how she has been acting and Lana bites her. Chloe calls Clark for help and he uses his super-hearing to find her. He arrives to catch Chloe just as she passes out, astonished to see Lana standing over her with a mouth full of bloody, pointed teeth. Clark takes Chloe to the hospital where the doctor reports that she has a rare strain of the rabies virus which is preventing her red blood cells from regenerating. They can treat her but they do not know the cure. Professor Milton Fine arrives at the hospital, claiming to be visiting a sick colleague. He asks about Chloe's condition and Clark admits that he thinks she was attacked by a vampire. Fine looks at Chloe's chart and tells him to ask Lex about Project 1138.
Back at the Tri-Psi sorority, the other girls are mad at Lana for leaving a victim alive. They tell her that Clark was snooping around and she has to clean up her mess by bringing him back so they can finish him off, or Lana will be the one they feed off instead.
Clark goes to the Luthor Mansion and demands that Lex tell him about the Project. Lex leads Clark into the library carrying a metal briefcase. He explains that six years ago, Buffy Sanders was trapped outside Smallville in a cave but was rescued by LuthorCorp. The cave was found to contain meteor rock-infected stalactites that had mutated the vampire bats living there. A LuthorCorp worker was also bitten and developed vampire-like symptoms like strength, fangs, and light sensitivity, but they were able to cure him. He opens the case to reveal vials of the serum, kept active by a large fragment of meteor rock. Clark starts to feel ill and backs away just as an overhead window breaks. Lana appears and jumps to the floor. She knocks Lex out and attacks Clark. Clark is weakened by the kryptonite and Lana bites him. After sucking his blood, she is able to shoot heat from her eyes. Clark is able to grab one of the syringes with the antidote in it before Lana takes him back to the sorority house.
The girls lay him on the hearth of the fireplace and prepare to feed on him. Lana does not want to kill him and suggests they just turn him into a vampire as well. The girls explain there is no room for him in a sorority. Buffy does not care what Lana wants and when Lana stands in her way, she knocks her to the floor. Buffy then tries to kill Lana, but she uses her heat vision to vaporize Buffy and the other vampires run away in fear. Lana prepares to turn Clark into a vampire but he is able to stab her heart with the antidote and restore her to normal.
Chloe's report for the Planet ends by saying all the girls were cured, including herself. Her roommate transferred, so Lana is now able to move in with her. She gives the article to Pauline Kahn, but Kahn claims that they do not print tabloid stories. Chloe counters with the research she did for the article including interviews, CDC reports, and eyewitness accounts. Kahn changes her mind, says that Chloe has potential, and agrees to hire her. Chloe is thrilled to start her dream career in the basement of the Daily Planet.