Saoirse Ronan and Beanie Feldstein play high school besties in “Lady Bird.” Photo by Merie Wallace, courtesy of A24

“I wish I could live through something,” the title character laments to her mother in the opening scene of writer-director Greta Gerwig’s Lady Bird. Played with comical intensity by Saoirse Ronan, seventeen-year-old Lady Bird — née Christine — is too young to realize that she is inescapably living through something, both in her own world and the wider one beyond Sacramento, California, the hometown from which she’s eagerly planning her getaway. A heartfelt coming-of-age story that perfectly captures the bittersweet transition from adolescence to dawning adulthood, Gerwig’s directorial debut is a joy from start to finish, a warm, generous snapshot of teenage vulnerability and exuberance.

Set in 2002, Lady Bird allows brief glimmers of the post–9/11 world order to break through the hermetic seal of Christine’s senior year, mostly through TV news broadcasts and in the inevitable mentions of terrorism anytime Christine mentions New York City — where she yearns to go to college. (If that doesn’t work out, she’ll settle for somewhere comparably cosmopolitan, “like Connecticut or New Hampshire.”). Gerwig has called Lady Bird a love letter to Sacramento, where she grew up, and she bathes the city in a golden, pre-dusk light — a testament to the mixed feelings many of us have for the places that made us, the towns you only realize you love when it’s time to leave.

Christine is a senior at Immaculate Heart (“Immaculate Fart,” she scoffs), with a face full of freckles and her shoulder-length brown hair dyed a fading pink. Ronan’s performance, which ranges from poignant to sidesplitting, is weighted with a real regard for the anguish of teenage life — she’s funny precisely because, like Lady Bird herself, she takes being Lady Bird so seriously. The high school drama in Lady Bird may seem trivial on the surface, tragedy made comedy from the distance of a decade. But Gerwig treats her young subjects as complex human beings, not clichéd broody teens.

Although Gerwig herself doesn’t appear, her giddy energy infuses both the film and character. A nun at Immaculate Heart (a delightful Lois Smith) tells Christine she has a “performative streak,” but that’s an understatement. Christine is ever committed to her role, whatever it is, and when you’re seventeen, it can change overnight. When she spots the first of her crushes, Danny (an adorable Lucas Hedges), at the grocery store with his family, she sidles up to him and delivers a line like a femme fatale in a noir: “Come here often?” Later, crushing on a boy (Timothée Chalamet) who plays in a band and reads Howard Zinn while smoking cigarettes, she steps into the role of the cool girl, blowing off the school play to hang out in a parking lot inexplicably nicknamed “the deuce.”

There are shades of the 2004 comedy Saved! in Lady Bird, with its Catholic school setting and sharp focus on high school hierarchies. Gerwig’s first act is a collection of funny, touching scenes of Catholic school life in Sacramento, “the Midwest of California,” as Christine calls it. She and her best friend, Julie (an excellent Beanie Feldstein), snack on communion wafers at school while giggling about masturbation; later, they audition for the school’s production of Merrily We Roll Along, a particularly hilarious scene in which Ronan adopts the affected Broadway trill of many a high-school drama hopeful.

The movie is also keenly attuned to the subtleties of American class; Christine loves to tell people she’s “from the wrong side of the tracks” (she is, literally), and when her mother, Marion, played by the wonderful Laurie Metcalf, takes her shopping, they go to the thrift store. To cheer her daughter up, Marion suggests they do “our favorite Sunday activity” — checking out fancy open houses. Her father, Larry (Tracy Letts), is the good cop to Marion’s bad cop, but he’s struggling, in a quieter way, with depression after losing his job.

Although Lady Bird maintains its focus on Ronan’s character, it’s in many ways Marion’s story, too, offering an insightful portrait of an intimate yet contentious mother-daughter relationship. Gerwig nails this dynamic, the subtle manner in which Marion’s little criticisms, small and sharp as a pin, poke into a daughter’s psyche the way only a mother can; or the way weeks’ worth of argument and hostility can drift off like mist when, on a shopping excursion, mother and daughter both spot the right dress at the same time.

Lady Bird moves fast, zippy and bubbly without sacrificing emotional weight. A decade in movies has given Gerwig a natural instinct for when to end a scene, often immediately after a punchline, the cut becoming part of the punchline, short, brusque scenes bumping abruptly into each other. Her vignettes of teenage life sometimes build to bigger plot points but sometimes just stand on their own — like that time you broke your arm, and you got a pink cast and scribbled “fuck you mom” on it, and then it healed, and it was like it never happened.

Lady Bird is a rare bird: sentimental without being saccharine, emotional without being contrived, able to conjure tears without yanking at our heartstrings while the music swells. Its matter-of-factness is what makes the film ultimately so wrenching. There’s no great tragedy here, and no great uplift; just life, as it’s actually lived, and the moments that make you who you are.