As Simon walks home from school, through the streets of Paris with his older sister Adele, they are both caught up in the excitement of the city. They stop by the grocer, the park, the natural history museum, a bakery, and an art museum. They even stop to watch a puppet show, a parade, and an acrobatic show.
Along the way they make new friends and meet old ones, and Simon loses one thing after the other. By the time they get home he no longer has his drawing of a cat, his books, scarf, crayons, gloves, hat, crayons, backpack, jacket, or sweater. However, they have had a day of happy adventures and pleasant memories in a world where friends, new and old, enjoy and help one another.