1980s, Quebec. Her name is Helene, but she calls herself Jo because she wants to be like Josephine La Petite Mousquetaire, her favorite cartoon heroine. Jo is only ten years old, has three sisters, a father who is very busy being unhappy and an understanding but strict mother. She lives in a working-class neighborhood populated by often colorful people, the most endearing of whom is her new neighbor, Monsieur Roger, a gruff old man who hides a tender heart. With her romantic soul and an imagination eager for great dramas, Jo would also like to achieve great feats. One day, she gives herself the mission of saving her father from work and unhappiness.