There was a very basic premise to this week's episode: at a Greendale costume party, Dean Pelton served old army rations from a military surplus. It was laced with some kind of biotechnology that made everyone act like zombies.
Pierce was one of the first to get bitten, but it appeared everyone else in the study group would make it out okay. But their barricade fell apart when Chang threw an ice skate (he was Peggy Flemming, of course) through a window and the zombie attack was on!
Abed sacrificed himself so Troy could escape, but this hero later returned and tried to save the day. He got bit, but he also made it to the thermostat and lowered the temperature, which saved everyone's life.
Eventually, the army made it to the school and used special devices to make everyone forget about the incident. Oh, Chang and Shirley also hooked up in the bathroom!