Superfudge by Judy Blume tells the story of twelve-year-old Peter Hatcher and his family as they grow by one member, Peter's baby sister, Tamara Roxanne, better known as Tootsie, and move from their New York City apartment to a large old house in Princeton, New Jersey.
Peter faces some major challenges at the beginning of the book. First, when his parents tell him that his mom is expecting a baby, he is horrified. He cannot help but wonder what will happen if the new baby is like Fudge, his four-year-old brother (whose full name is Farley Drexel Hatcher). Fudge is a nuisance to say the least, with his wild ways and his tendency to stick his nose into all of Peter's business. Peter decides he will run away, but his parents get him to agree to at least see what the new baby will be like.
After the baby arrives and turns out to be pretty cute after all, Peter's parents announce that they are moving the family to Princeton so his dad can pursue his writing career. Peter is upset, especially at the thought of leaving his best friend, Jimmy, but again, he agrees to give their new home a try.
After moving to Princeton, Peter quickly becomes friends with Alex Santo. Fudge continues to be his annoying, embarrassing self, and Peter is worried when his parents tell him that although Fudge is a bit young, he will be starting school in the fall. Sure enough, on the first day of school, Peter is called to the principal's office to help with Fudge, who has reacted badly when his teacher refused to call him by his nickname. Peter gets Fudge down from the top of the storage cupboard, settles him in the other kindergarten class, and laments that his brother's school career has begun.
When Fudge gets a bike for Christmas, Peter gets the job if teaching him to ride it, and that is no easy task. As the year in Princeton continues, Peter is often embarrassed by his brother and frustrated by his parents, all the while dealing with the emotions of growing up, including liking a girl named Joanne McFadden, who gets the task of drying off Peter's back when Fudge spills his drink at the movie theater.
When spring comes, Fudge, angry that he has been excluded from Peter and Alex's picnic, goes off with his friend Daniel for an adventure of their own. Peter's parents and Daniel's mother are terrified when they can't find the boys, but finally Fudge calls to say that they went for lunch. Peter suggests a proper punishment: the boys can't ride their bikes for a month.
As the school year and the story draw to a close, the family is faced with the decision of whether to stay in Princeton or move back to New York. They decide to go back to the city, and even Tootsie ratifies the choice with a firm “Nu yuck!”