The narrator is Jill Brenner, a Pennsylvania fifth-grader[1] who joins her classmates in ostracizing and bullying Linda, an awkward and overweight girl.

Linda gives an oral class report about whales and is hence nicknamed "Blubber" by her peers.

On Halloween, Jill and her friend Tracy play a prank on their grouch of a neighbor, Mr. Machinist, by stuffing raw rotten eggs into his mailbox.

However, they are later identified from a photo taken by Mr. Machinist and are made to rake leaves in his backyard as punishment.

Jill comments on how friendships in the class have changed completely, but notes that Tracy is a friend she can always count on having.

According to Judy Blume in a short essay published in a recent reprint of Blubber, the plot was inspired by a similar real-life incident involving her daughter's 5th grade class.

"My daughter was the shy, quiet girl in the class, the observer, like Rochelle", Blume wrote.

Blume also explained that she wrote Blubber to encourage children who see bullying taking place, as well as the victims themselves, to tell someone they trust rather than keep it to themselves.