In the story, Mrs. Rudder comes to Fudge's birthday party by accident. Because she lives in the apartment downstairs, she can hear all the commotion above her.
Meanwhile, up at the Hatcher apartment, Peter (Fudge's brother) has just blown up a lot of balloons. He hands each of Fudge's little guests balloons and then proceeds to dance around with his balloon. Fudge's friends, Ralph, Jennie, and Sam, soon follow suit. All the children dance around with their balloons to the record player's tunes. They even jump on the furniture and run around the apartment; in general, they make so much noise that Mrs. Rudder is forced to come up and investigate the reason for all the ruckus.
In fact, Mrs. Rudder tells Peter's mother that it felt as if "her ceiling was about to crash in on her any second." Meanwhile, Peter's mother explains that Fudge is having a birthday party. She invites Mrs. Rudder to stay and to eat a piece of birthday cake. This is how Mrs. Rudder comes to attend Fudge's birthday party.