Christine McPherson is a Sacramento, California, Immaculate Heart of Mary Catholic High School senior in the fall of 2002. And she hates being one. She refers to herself as "Lady Bird." Christine McPherson and her mom, Marion McPherson, drive the car. Despite her family's financial difficulties, she yearns to enroll in a top college in "a city with culture" somewhere on the East Coast. Christine's mother, Marion, dismisses her hopes for the future and accuses her of being ungrateful for what she now has. Following an altercation, Christine jumps from a moving automobile, fracturing her arm.
When Christine and her closest friend Julie enroll in their school's theater program. They stop to admire a blue house on the way home. Her mother is a doctor, and her brother Miguel is a grocery store employee who dates Shelly. Christine speaks to her father about her college and expresses a desire to enroll at a school on the East Coast. And Mr. Larry McPherson says he is going to look into the options. Christine and Julie are chosen for the theater program for which they applied.
Christine starts dating Danny, a student at the nearby boys' school. After their first kiss, Lady Bird sneaks inside her house on prom night and is told by her mother to act appropriately because her father lost his job. Since she received a poor grade, Lady Bird snatches and throws the math teacher's grade book in the trash. When Danny brings Lady Bird to his grandmother's house, she discovers that it is the same beautiful blue mansion that they both like.
To Marion's dismay, Christine spends her final Thanksgiving before graduating with Danny's affluent family rather than her own. After the school performance of Merrily We Roll Along on opening night, when Christine discovers Danny kissing a boy in a loo stall, they decide to call it quits.
As a Christmas gift, her father presents her with a financial aid application for East Coast College. Christine surreptitiously submits applications to institutions on the East Coast with the assistance of her father, despite Marion's protestations that their family cannot afford the costs. Christine starts working at a coffee shop at Marion's urging, and there she meets Kyle, a popular lad at the school.
To connect with popular student Jenna Walton, Christine forgoes auditions for the upcoming school play. Together, they vandalize a nun's car. When Danny sobs hysterically and explains his anxiety about coming out, she comforts him, and they rekindle their friendship. UC Davis accepts her, but she thinks the campus is too close to her house.
Christine and Kyle kiss during a house party, and both admit they are virgins. Christine finds out that Larry, her father, has been unemployed and dealing with depression for many years. Christine starts to spend less time with Julie as she gets to know Kyle and Jenna better and quits the theater program. Lady Bird is expelled from school for speaking up rudely during an anti-abortion assembly. When Marion hears this, she reprimands her and tells Lady Bird how difficult it is to raise a child on a fixed income. Lady Bird is upset by this and asks for the money to repay her mother later. Marion claims that she worries about Lady Bird's capacity to find a job and make a repayment.
When Jenna tries to visit Lady Bird after her suspension, she learns that Lady Bird had pretended to be at Danny's grandma's house as hers to impress Jenna. Because of their shared friendship with Kyle, Jenna agrees to accept her apology for the deception, but their relationship deteriorates. After Lady Bird’s first sexual encounter, Kyle admits that he was not a virgin before they had sex, which upsets Lady Bird and makes her turn to her mother for consolation. And she takes Lady Bird for Sunday activities.
She subsequently finds out that she is waiting to enroll at a university in New York City, but she keeps this information to herself and her dad. Larry and Miguel go to an interview for the same job at a corporation, and Larry encourages Miguel to get the job that he was unable to. Sarah, Lady Bird's sister, has stated that she won't be punished for abusing her vehicle. She also makes an effort to convince her that Lady Bird still has feelings for Sacramento.
Lady Bird and her mom go prom shopping. Kyle, Jenna, and Jenna's boyfriend leave with Lady Bird for the prom, but the latter three decide to attend a house party instead. At first, Lady Bird accepts but later requests that they drop her off at Julie's, where they two mend fences and attend prom together.
After graduation, Danny mistakenly brings up the wait list in front of Marion, causing Marion to cut off communication with Lady Bird for the remainder of the summer. When Lady Bird is accepted to the university, her parents drive her to the airport, but Marion won't enter to bid Lady Bird farewell. When Marion returns from the airport after crying while leaving it and finds Lady Bird has already passed through security, she is disappointed. In Larry's arms, she sobs, and he comforts her.
Lady Bird discovers many letters addressed to her in her suitcase after arriving in New York. Her mother wrote the letters, which she initially threw away, but her father secretly kept them and gave them to her. She consumes too much alcohol at the party and ends up in the hospital. She then returns to using her first name.
She visits a Presbyterian church service after leaving the hospital and is brought to tears. When she calls home, she apologizes and thanks her mother for everything she has done for her and admits that the name they gave her is the good one.
