Told in seven short chapters, The Hundred Dresses begins on a Monday morning in November when Wanda Petronski is absent from school. At first, no one in Room 13 notices that Wanda is gone.
Wanda's desk is at the back corner of the room; she is characterized as quiet and unremarkable. She lives in Boggins Heights, an impoverished area on the outskirts of the Connecticut town where the narrative is set. The narrator reveals that Peggy and Maddie often wait for Wanda outside the school to taunt her.
Peggy and Maddie finally notice Wanda's absence on Wednesday, because they waited for her in the schoolyard. The girls rush to their seats as the class is finishing reciting the Gettysburg Address, a famous speech about the American ideal of equality.
The narrator explains how Wanda's Polish surname sets her apart from her classmates, who have Anglophone names that are easier to pronounce. Wanda is friendless. Her only contact with the other girls is when they wait to tease her about the hundred dresses she claims to have lined up in her closet at home. The girls believe this to be a lie, since Wanda only ever wears an old blue dress.
Wanda never seems bothered by the taunts. She simply stands alone against an ivy-covered wall and waits for the school bell to ring. Maddie is nonetheless remorseful about how she stands by while Peggy teases Wanda. Maddie herself wears Peggy's old dresses that Maddie's mother disguises with new trimmings. She worries that if she stood up for Wanda, she would become the new target of schoolyard bullying.
Maddie remembers the day the hundred dresses game began. It was a bright day in October, and Cecile was showing off her new red dress. All the girls were gathered on the street, admiring the dress. Wanda arrived with her brother Jake, who had to walk ahead without her and perform odd jobs at the school. Trying to join the discussion about dresses, Wanda told Peggy that she had a hundred dresses at home. From that day on, Peggy would taunt Wanda about the dresses.
Maddie writes a note to Peggy to say that she should stop taunting Wanda, but Maddie rips it up when she remembers how she herself could become a target. She reasons with herself that Peggy is the most popular girl in school, so she must be virtuous, while Wanda is basically a nonentity. While thinking about the classroom drawing contest, Maddie forgets about Wanda.
The next day, a Thursday, it is pouring rain. Inside Room 13, Peggy and Maddie marvel at the hundred drawings of dresses pinned to the walls. Miss Mason, their teacher, announces that Wanda submitted one hundred drawings of different dress designs, and that she won the contest. Just as the class is reconsidering their impression of Wanda, Miss Mason receives a letter from the principal's office. The letter is from Wanda's father, and it says that Wanda and her brother and father are moving to the city, where they won't be teased for their ethnic difference as Polish people.
A bleak mood follows, and Maddie is filled with remorse. After class, she and Peggy go to Wanda's house in Boggins Heights to find it empty. Unable to apologize in person, the girls write a letter and address it to the old address with a request to forward it to the Petronskis' new home.
Having felt better, Maddie's remorse nags at her while she waits for a reply. To help herself sleep at night, Maddie imagines scenarios in which she defends Wanda from harassment and danger.
On the last day of school before the Christmas break, Room 13 receives a letter from Wanda. Miss Mason reads it aloud: Wanda asks that the drawings be given to the class as gifts. She specifies that two particular drawings should go to Peggy and Maddie. Wanda's kind gesture reassures the girls.
At home, Maddie pins the drawing over a torn patch of her wallpaper. She still wishes she could make up with Wanda in person. While staring at the drawing, she notices the figure wearing the dress is actually a portrait of her. In her excitement, she rushes to Peggy's house and shows Peggy that her drawing is also a portrait. The girls conclude that even though they were cruel to Wanda, Wanda must have admired them.
The book ends with Maddie tearfully recalling the image of Wanda standing alone against the ivy-covered wall in the schoolyard and looking over at the girls who made fun of her.