Set in Sacramento during the early 2000s, the story follows Christine McPherson, a high school senior who insists on being called "Lady Bird." She dreams of leaving her mundane hometown and attending a prestigious college on the East Coast. Her mother, Marion, is practical and overbearing, constantly clashing with Lady Bird over her aspirations and the family's dire financial situation. Lady Bird's father, Larry, is more supportive but struggles with his own issues, including unemployment. As she navigates her senior year, Lady Bird grapples with friendships, romantic relationships, and the looming pressure of college applications.

Lady Bird attends a Catholic school where she and her best friend Julie join the theater program, only for the friendships to face strains due to changes in their personal lives and aspirations. Lady Bird also explores relationships with Danny, a charming boy from the theater program, and later Kyle, a brooding musician. Each relationship teaches her more about herself and what she values in life. Amidst the chaos of her final year of high school, Lady Bird's tumultuous relationship with her mother remains at the forefront, highlighting their differences but also their deep-seated love for each other. As graduation approaches, Lady Bird must confront her realities, relationships, and dreams in her quest for self-discovery.