Peter Hatcher is a nine-year-old 4th grader living in New York City with his family: his mother, his father, and his almost three-year-old brother, Fudge. Peter calls Fudge his biggest problem because Fudge is constantly causing trouble, misbehaving, and making Peter's life difficult. Each chapter of this book tells the story of one of Fudge's antics.
One day, Peter wins a small turtle at his friend's birthday party. He is very proud of it, and names it Dribble. He promises his parents he will take care of it on his own, and keeps it in his room away from Fudge. Not long after, the president of Juicy-O, Peter's father's advertising client company, comes to visit New York with his wife and stays with the Hatchers. Fudge misbehaves over and over again, causing the guests to believe the Hatchers have not taught their children manners. They leave the house, and Peter's father loses the Juicy-O account, but he is not too disappointed about it.
After this, Fudge goes through a phase where he will not eat anything. At first he pretends to be a dog and will only eat food off the floor, but following that, he refuses to eat at all. Peter's mother worries and takes him to many doctors, who all say there is nothing wrong with him and he will eat when he is hungry. Finally, he eats after Peter's father tells him, "eat it or wear it!" and dumps a bowl of cereal over his head. Another day, Peter and his friends are put in charge of babysitting Fudge for a few minutes in Central Park, but when they are not looking Fudge jumps off the jungle gym and knocks out his two front teeth.
Fudge's third birthday arrives, and they invite three of his little friends from the neighborhood over for a short party. The party is complete chaos, with one child who bites, one who eats too much, and one who is afraid of everything. Peter and his mother are relieved when the party comes to an end. Then one Saturday, Peter's mother takes the boys out to run errands, and multiple times Fudge proves that only Peter's example will get him to behave. The dentist makes it a competition between him and Peter to get him to open his mouth, and the shoe salesman has Peter pretend to get the same shoes as Fudge so he will agree to try them on.
Peter's class starts a project on New York City at school, and his group is assigned to research transportation. The project is almost done when Fudge gets into Peter's room and scribbles all over his poster, ruining it. A few weeks later Peter's mother travels to Boston to help with her sister's new baby, so the boys are left alone with their father. He takes them to his office on a day Peter has off from school, and Fudge is inadvertently chosen as the child actor for the new Toddle-Bike commercial. Peter is angry that Fudge will be famous, while he is just a "fourth grade nothing." Fudge refuses to ride the bike at first, but then they ask Peter to demonstrate for him and he does it, wanting to be like his brother. Then they go to the movies, where Fudge gets lost in the theater. They do not tell their mother what transpired during her weekend away.
But worst of all is the day Peter comes home from school to find out that Fudge has swallowed his turtle, Dribble. They rush Fudge to the hospital, and the doctors give him medicines that will help the turtle move through his digestive system quickly. Finally the turtle comes out, and Peter is distraught that Fudge has killed his pet. Finally, though, Peter's parents recognize him for handling the situation so maturely, so they buy him a puppy of his very own, which he names Turtle.
