Location: The Greendale Library. Event: Greendale Halloween Party. Dressed as David Beckham in a $6,000 suit, Jeff greets Pierce, who's dressed as Captain Kirk. Pierce is thoroughly enjoying the party food and compliments it just as Dean Pelton, dressed as Lady Gaga, walks up. Jeff calls the Dean out on his not-so-chic choice of catering for the party. Apparently the dean got the food from an army surplus food store. Britta, dressed as T-Rex, and Annie, dressed as Little Red Riding Hood, discuss Shirley's costume. Last year, everyone thought she was Urkel when she was actually Harry Potter. This year, her costume is just as ambiguous. Annie alerts Britta and Jeff that whatever Shirley is, she's definitely not Miss Piggy.
Pierce starts to sweat and feel sick at the party. Things turn ugly very quickly as Pierce freaks out. Annie lends him a helping hand down the library hallway. She asks a student dressed as a doctor for help, but he's not an actual doctor, it's just a costume. Then, a guy in a banana costume says he's a doctor. Annie is delighted; to top it off, the doctor is Rich from pottery class. Dr. Rich gets Pierce into a chair and starts examining him.
Abed and Troy join the party in their robot costumes, which they spent days making. Dean Pelton brings Dr. Rich a few more patients that appear to be getting sick. Dr. Rich suggests that with fevers running at 102, they might be looking at something more than food poisoning. Just when Dean Pelton asks him not to speak too loudly, Pierce bites Starburns' arm as he walks past. After witnessing this bizarre attack, Dr. Rick officially rules out food poisoning.
Jeff tells Britta that the vibe at the party is quickly getting creepy. Almost one in every five people looks like they're totally high on drugs. Across the room, Troy walks up to Abed with a new costume, having decided to go it alone. Troy is now a sexy Dracula instead of a robot like Abed. Abed tries to explain that if Troy doesn't have his costume on, Abed's doesn't make sense. Troy walks away, calling Abed a nerd, and hurting his feelings.
Dean Pelton leaves the party and heads back to his office. He immediately calls the surplus store where he purchased all the food for the party. When he reads off the label number to the person on the other end, silence ensues. Dean Pelton is transferred to Special Operations Officer McHenry from the U.S. Army. Back at the party, Leonard attacks a woman by biting her neck, just like Pierce's attack on Starburns. Troy and Abed call it like they see it. It's a zombie attack.
Just as everyone starts to run from the zombies in the library, Dean Pelton closes all the doors leading to the exit. He has been instructed by the army to quarantine the partygoers immediately. When they realize they're trapped, Jeff instructs everyone to start running to the back. Chang tries to lock himself into the study room, but Jeff manages to get Annie, Britta, Troy, Abed, Shirley and Rich into the study room to safety. Before they barricade the door, Dr. Rich asks if anyone's been bitten. Then he explains that the infected don't have six hours to wait for the Army. The temperature in the library is too high. With fever, brain damage will be followed by death. Annie suggests lowering the temperature in the building. Dr. Rich thinks that might just work.
Abed enlists Troy's help to get to the thermostat outside the study room. Troy doesn't want to help, reminding Abed he's no longer a robot, he's sexy Dracula. Dr. Rich explains the zombie symptoms. When he begins to slur his speech, it's clear he's been infected. Then Britta shows that she was bitten too. When Britta and Dr. Rich start to turn into zombies, Chang throws an object at them hoping to knock them out, but misses and breaks the study room window letting the zombies from outside into the room. Annie's the first to get attacked.
Jeff, Shirley, Troy, Abed and Chang run out of the study room away from the zombies. Shirley and Chang lock themselves into the bathroom where they comfort each other. Jeff, Troy and Abed make their way to the basement, where Abed confronts Troy - why did he change his Halloween costume? Troy tries to explain he did it because he's not a nerd.
Zombies attack Jeff, Troy and Abed in the basement. Jeff goes down while Troy and Abed still have a chance to get out. Abed helps Troy escape. Now he's the only one who can help the study group. Troy gets the keys from Dean Pelton, puts his robot costume back on, and heads back into the zombie-filled party. He needs to get to the thermostat before getting sick, but Abed bites him. Luckily, Troy manages to reach the thermostat anyway. Minutes later, the air turns on, and everyone starts to wake up from their zombie trances.
The Army arrives six hours later on the dot, followed by the police and medical teams. Declared healthy, Troy and Abed head back to the dorm for a movie night. They're best friends once again.