Basically, Annie can’t find her pen. It is a purple gel pen, and it is the eighth one she has lost during group study sessions. She gets very upset and starts screaming. Because it’s not about the pen—it’s about the principle! Someone is taking her stuff and it is very inconsiderate!
So instead of living their lives—which in this case means going to Greendale’s puppy parade, which obviously wasn’t featured on the show because you can only see a puppy parade in heaven—everyone stays in the study room and looks for Annie’s pen. They search each other’s bags. They get naked. They turn on each other, and then they turn on each other but in a slightly different way. Abed reveals that he has been tracking Shirley, Britta, and Annie’s menstrual cycles, and with shocking accuracy!
Basically, it’s a bottle episode. I know because when Jeff cancels his date to stay and look for the pen, he tells the anonymous woman he was about to go out with, “Tell your disappointment to suck it! I’m doing a bottle episode!”
And then he throws his phone across the room.