While walking down the stairs outside 123 Sesame Street, Cookie Monster slips on a rollerskate, and falls and hits his head. David and Maria come to his aid only to discover that Cookie Monster has lost his memory. He doesn't know who he is, and can't recognize his friends.
Cookie Monster doesn't know who he is. After meeting Big Bird, he thinks that he may be a furry bluebird. But after trying to eat a dish of birdseed and not liking it, he decides that he's definitely not a bird.
Cookie Monster wonders if he's a mailbox. Like the mailbox, he's blue, and he has an opening in front. But his attention is drawn to the fact that a mailbox is made of metal, not fur, and that a mailbox isn't alive. Cookie Monster agrees that he's not a mailbox.
Cookie Monster states that he's not a Grouch, not a Bird, and not a mailbox. Maria tells him that he's a very nice monster, and asks him to play a game with her. She takes out three large cookies and one small cookie, and sings "One of These Things". After classifying the cookies by size, Maria convinces Cookie Monster to eat one. But even the taste of the cookie doesn't bring his memory back.
Betty Lou teaches the meaning of "beginning", "middle", and "end", and introduces two of her friends: Beginning and End. The duo tell a story, but it lacks something in between the beginning and end parts. Beginning and End recruit their pal Middle to fill out the rest of the story. Middle, however, cannot stop telling the exciting part of the story, and the three friends begin a quarrel, forcing Betty Lou to declare the end of the lecture. (First: Episode 0138)