Lady Bird Saoirse Ronan, Laurie Metcalf, Beanie Feldstein. Directed by Greta Gerwig. Rated R. Opens Friday at Downtown Summerlin.

Greta Gerwig’s solo writing and directing debut follows many of the familiar beats of the teenage coming-of-age story, but Gerwig gives it a personal specificity that sets it apart from the many unremarkable indie teen dramedies released every year. Lady Bird’s title character is the kind of role Gerwig herself might have played a decade ago (and indeed the movie is at least partially inspired by Gerwig’s own teenage years), but Saoirse Ronan capably fills in for the filmmaker while making the performance entirely her own.

A high school senior in 2002 Sacramento, California, Christine “Lady Bird” McPherson (she insists everyone, including her family members, call her by that self-applied nickname) is an underachiever who thinks of herself as a misunderstood genius, even though there’s little to indicate she possesses untapped talents. She treats her family, her Catholic high school and her hometown (which she refers to as “the Midwest of California”) with equal contempt, but Gerwig and Ronan make sure that Lady Bird is never unlikable, just a bit naïve and self-centered, like most teenagers.

The movie follows Lady Bird’s final year of high school, with typical teenage milestones: She dates boys who are wrong for her; she fights with her mother (an excellent, understated Laurie Metcalf); she bonds with and then ditches and then makes up with her best friend (Beanie Feldstein); she applies to colleges that are probably out of her reach. Gerwig presents it all with a low-key authenticity and a sharp (but not unrealistic) wit. Lady Bird’s journey toward understanding her own limitations and her true potential isn’t overblown, with Gerwig finding humor and drama in small everyday moments. Lady Bird is only beginning to mature by the time the movie ends, but as a filmmaker, Gerwig proves that she’s already there.