Christine "Lady Bird" McPherson, portrayed by Saoirse Ronan, is a high school senior navigating her final year at a Catholic school in Sacramento during the year 2002. Living with her parents, she is often at odds with them, alongside her older brother, who is adopted, and his girlfriend. Lady Bird shares an unshakeable bond with her best friend, Julianne "Julie" Steffans, and together, they dive into their school's theater program. It is there that Lady Bird encounters Danny O'Neill, played by Lucas Hedges, sparking a romance that even leads her to choose Danny's family Thanksgiving dinner over her own.
However, this budding relationship takes a tragic turn when Lady Bird discovers Danny kissing another boy in a bathroom stall, shattering her heart. At the insistence of her mother, Marion McPherson, Lady Bird starts a job at a coffee shop, where she meets the edgy and charismatic Kyle Scheible. As their romance blossoms, Lady Bird starts drifting away from Julie, and instead, befriends popular girl Jenna Walton. After an incident involving a reprimand from nun teacher Sister Sarah Joan for Jenna's attire, they find a sense of camaraderie in mischief.
In a moment of unfolding truths, Lady Bird mistakenly tells Jenna she lives at Danny’s grandmother's house, which contributes to the unraveling of their friendship. Following a heart-wrenching encounter with Danny, who confesses his struggles with identity, Lady Bird's innocence is further tested when she loses her virginity to Kyle after being misled about his own experience.
Amidst her family's turmoil—her father's job loss and battle with depression—Lady Bird applies to out-of-state colleges against her mother's wishes. Despite numerous rejections, she receives a glimmer of hope with a spot on the waitlist for a university in New York. As prom approaches, Lady Bird joins Kyle and his friends but ultimately chooses to reunite with Julie, a step toward reclaiming a crucial friendship.
With her driving test passed and her room transformed, tensions rise when her mother discovers she has secretly applied to colleges. In a bittersweet turn of events, on her 18th birthday in 2003, Lady Bird shares a cupcake with her father, who humorously suggests their family is too broke for divorce.
As she embraces adulthood, Lady Bird sets off for New York City, but not before a cold goodbye with her mother, who masks her tears. Unexpectedly, Lady Bird’s journey pivots when she finds heartfelt letters from her mother, salvaged by her father, prompting her to reclaim the name Christine. A spell in the hospital after a wild party leads Christine to reflect on her life, and outside a church, she courageously leaves an earnest message for her mother—a step towards reconciliation.
