The study group along with the rest of the student body are attending the Halloween party in the library. Pierce starts to become sick after he eats the food there which turns out to be military rations from an army surplus store. Annie helps Pierce get medical attention from Rich who is also in attendance. Elsewhere, Troy and Abed are putting on a show in their costumes for the benefit of two female students who scoff at their antics. When Troy asks Jeff why they failed to impress them, he points out their immaturity was a turn off. The Dean sends Rich more sick students to look after. Rich's preliminary diagnosis of the sickness is undermined when Pierce suddenly bites Star-Burns' arm.
Troy decides to ditch his costume, going for a more mature and sexy look which confuses Abed. Star-Burns starts exhibiting the same symptoms as Pierce despite not having eaten the army rations. Annie suggests to Rich that Pierce transferred his sickness via the bite. Star-Burns ultimately loses it and tries to attack them. In another room, Dean Pelton is talking on the phone with the food supplier. When he mentions key phrases written on the ration containers, a government agent intercepts the call. Rich and Annie arrive at the study room and warn Britta, Shirley, and Jeff about the contagion. At that moment, Leonard bites another student on the neck, creating a huge panic.
Everyone makes a run for the exits as the other infected students begin to attack as well. The study group gets to the library entrance but finds that Dean Pelton is locking them all in. He tells them they have to be quarantined until the army arrives which will be six hours from now. The group is forced to make their way back to the study room. Jeff tries unsuccessfully to contact the authorities while Troy and Abed start to barricade the room. Rich explains the infection and high fever the students are experiencing will lead to brain damage with 3 hours and death within six. Annie suggests that they counter the fever by lowering the temperature of the building.
Abed volunteers himself and Troy to get to the library thermostat to make it colder, but Troy refuses. He insists on staying in his new mature persona and doesn't want to risk his life. Rich confirms how risky it is to get infected and starts listing various symptoms of the virus including slurred speech. Rich suddenly starts to talking in a slurred manner as does Britta, revealing them both to be infected. The duo starts to attack the others, causing Chang to throw one of his skates at them. The shoe ends up hitting one of the study room windows, destroying it and allowing a number of infected students to grab Annie and pull her out of the room. More zombified classmates enter through the window as the remaining survivors flee.
They quickly exit the study room and make their way past another group of infected students. Shirley stumbles and falls, but Jeff insists to the others to keep going. Chang stops to help her, and they both manage to make it to the restroom. As they try to hold back the horde of zombies bearing down on the door to the restroom, the two share a moment of understanding and kiss. Meanwhile, Jeff, Troy, and Abed successfully make it to the library's basement. Abed confronts Troy over his new attitude, but the latter refuses to talk about it. Jeff is then startled by a noise which turns out to be just a cat. Initially relieved, he soon becomes annoyed when it repeatedly jumps in and out of their way.
After chasing it, the trio come across a possible exit in the form of a window that leads to the outside. Jeff instead opts to leave out of a door on the far side of the room. When he opens it, there are infected students behind it including Rich who immediately pounce on him. Troy and Abed escape the horde by climbing and crawling across metal shelving units. They reach the fenced off section in front of the window and barricade it. Seeing that the window is too high to reach and knowing the zombies will break through soon, Abed offers Troy a boost. He is reluctant to accept, but Abed insists, telling him that he is willing to sacrifice himself for Troy's survival. With Abed's help, he reaches the window and the outside but is forced to watch as the zombie students overtake Abed.
Troy makes it to the outside and confronts the Dean. After Pelton refuses to hand over the keys to the library, Troy punches him and takes them. Before entering the building, he puts on his original costume for more protection, but it proves ineffectual as the zombie students dismantle it easily. Slipping away from them, he makes his way to the thermostat. Along the way, he comes across the rest of the infected study group members. He manages to make his way past them but pauses when he runs into Abed who bites him. The others join in, but Troy shrugs them off and struggles to reach the thermostat as the infection starts to take hold. With his last bit of strength, he manages to turn the temperature down before turning into a zombie.
Troy and Abed survey the aftermath of the Halloween party.
The temperature in the starts to drop, and the students inside slowly start to regain their senses. Outside, the army finally arrives and see that the students have overcome the infection. Instead of cleansing the area they decide to dose the students with knockout gas. Some time later, police officers led by Officer Cackowski are at the library along with the E.M.T's. Cackowski is interviewing the Dean, but like the rest of the partygoers, he has no recollection of the night's events. It's revealed a cover story was created explaining that everyone had been roofied. Troy and Abed decide to leave and go watch a movie. For reasons unknown, they both agree to avoid any film that contains zombies in it.
Previously: It's mentioned on Britta's Campus Connect profile as well as the Season One episode "Pascal's Triangle Revisited" that she had a bad experience with a man in a dinosaur costume and in this episode she wears a T-Rex outfit.
This must be the place: The story takes place in the library.
Middle Eastern Magic 8 ball: This episode sets up the story arc between Shirley and Chang and also one between Annie and Rich.
Identity Crisis: Troy realizes how much he has changed from a jock to a nerd due to his friendship with Abed and tries to adopt a more mature persona.
Double take: Chang repeats the phrase "Ya bit? Ya bit?" which he used back in his first appearance in the Season One episode Spanish 101.
A nice gesture: Troy and Abed do their signature handshake while they are both zombies.
Familiar face: Officer Cackowski shows up at the end of the episode to investigate what happened at the party.
Discontinuity: Even if the government managed to erase all the students' memories of the night, there would still be plenty of evidence of what happened just from all the injuries the students suffered, particularly the bite marks. Also, despite claims that no one remembers anything about that night, Abed somehow later recalls in "Paradigms of Human Memory" an incident involving Jeff and Britta. Chang somehow managed to have sex with Shirley, take pictures of it with his camera phone, and send a message to Troy's phone all while holding back the restroom door that a horde of zombies was beating down upon.
Nice outfit: Jeff is dressed as David Beckham, Britta is dressed as a Tyrannosaurus Rex, Annie is dressed as Little Red Riding Hood, Shirley is dressed as Glinda the Good Witch, Pierce is dressed as Captain Kirk, Troy is dressed as Ripley in lift suit and later as "sexy Dracula", Abed is dressed as the Alien, Chang is Peggy Fleming, and Dean Pelton is dressed as Lady Gaga.
NOOOOOO!: Troy screams this after watching Abed being taken down by a group of infected students.
Name that tune: Much of the soundtrack of the episode is comprised of ABBA songs from Dean Pelton's playlist. "Dancing Queen" and "Fernando" in particular are featured prominently in this episode. Though not in the episode, Jeff refers to Public Enemy song "911 Is a Joke" when he says "Flavor Flav was right" after attempting to dial 911 and not getting a hold of anybody.
TV Guide: The Oscars running time is made fun of by Jeff.
Parody: This episode serves as a parody of various zombie and horror movies, with the infected students mimicking stereotypical zombies and various tropes of the genre being mocked.
Résumé: In this episode, Pierce is dressed as Captain Kirk from "Star Trek". This is not the first time Chevy Chase has dressed as a character from the series; in his stint on SNL, he played Spock in a famous skit about Trek's cancellation.
Karey Dornetto, Community producer and writer of "Epidemiology", took photos with the cast inbetween production. She later posted the photos online at her website.
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Added by Ultimateatomicbuster
