Lady Bird, Greta Gerwig’s endearing coming-of-age portrait, might not have made it out of urban art houses and into the mainstream multiplex if it hadn’t been for some new math. The indie, with its modest ($10 million) budget and a first-time director, made national news when it achieved a rare and coveted 100% “fresh” rating on Rotten Tomatoes, the popular movie review aggregation site. This officially surpassed big-budget blockbuster Toy Story 2 (1999), which also had a 100% rating but from slightly fewer critics.

Gerwig is best known as a quirky but intense actress of the so-called “mumblecore” genre. With a focus on the everyday trials and tribulations of Gen Y, these films feature pedestrian situations and dialogue, delivered with little fanfare, some mumbling, and no special effects. Typically, they attract attention at film festivals and inspire small but passionate college-town communities of fans. Since graduating from Barnard College, she has appeared in (and in some cases co-written) about three dozen films, most of which you probably haven’t heard of. However, she received a Golden Globe nomination for taking on the title role in 2013’s charming Frances Ha, and drew enthusiastic notice in Jackie, Maggie’s Plan, and 20th Century Women.

For Lady Bird, her first solo effort behind the camera, Gerwig draws from personal history. Christine “Lady Bird” McPherson is an intelligent but disenchanted high school senior, growing up in Sacramento in the early 2000s. She lives on “the wrong side of the tracks,” with a nurse mother, an out-of-work father, her slacker brother, and his girlfriend. She attends Immaculate Heart Catholic school (because, as her mother reminds her constantly, her brother “saw someone stabbed at Sac High!”). She longs to escape. “I hate California, I want to go to the east coast. I want to go where culture is like, New York, or Connecticut, or New Hampshire.”

Gerwig admits that her inspiration is autobiographical, but claims to have had little in common with her lead character. In an interview with IMDB, she laughingly asserted that “I had nothing embarrassing happen to me in high school. I’m the only person in the world [who didn’t]. Everybody liked me and I never made a mistake … that’s why I had to make this all up.”

As idyllic as Gerwig’s own youth may (or, apparently, may not) have been, the fictitious Lady Bird has issues. The teen as square-peg-in-a-round-hole set-up has been leveraged by Hollywood for decades, as anyone who has ever sat through a John Hughes film will attest. Like other high school heroines before her, Lady Bird struggles with insecurity, ambition, self-esteem, and the opposite sex. But Gerwig has achieved the near-impossible, she has created a delicious young character whose situation is at once completely familiar and uniquely unusual.

Aside from her terrific screenwriting and sensitive direction, Gerwig is aided by a remarkable performance from one of today’s most talented young actresses. At 23, Saoirse Ronan (it rhymes with Inertia, as she pointed out recently on Saturday Night Live) has already received two Oscar nominations, for 2004’s Atonement and last year’s Brooklyn. Another nomination — and perhaps even a win — for her turn here won’t be unexpected.

In fact, earlier this week it was announced that Lady Bird, its screenplay, Ronan and her costar Laurie Metcalf are all in the running for a 2018 Golden Globe.

Metcalf plays Lady Bird’s mother, Marion. (She previously won three Emmys for her work on Roseanne and has already been honored by the Boston Society of Film Critics for Lady Bird). The tempestuous scenes between mother and daughter are absurd but heartbreaking. There are moments when the bond between them is so strong it shines, like when they find the perfect dress at the local thrift shop. And others when neither pulls her punches. Impossibly hurtful things are not just said but hurled across the room or the car’s front seat like a weapon of maternal destruction. However, when her first fumbling boyfriend Danny (Lucas Hedges) suggests, “Your mom’s really hard on you,” Lady Bird shrugs. “Well, she loves me a lot.”

Lady Bird’s father Larry is played by Tracy Letts, Tony and Pulitzer Prize winner and last seen onscreen in The Lovers. He has a special bond with Lady Bird; in truth, they’re both a little scared of Marion. But, like so much of the film, Gerwig doesn’t make him a stock character. He has his own terribly real challenges, like trying to convince a smug millennial to hire him.

The compassion Gerwig feels for all her characters is at the very heart of Lady Bird. Marion could be a monster, but she isn’t. Larry is beaten down but remains optimistic. Their son and his girlfriend (Jordan Rodrigues and Marielle Scott), with multiple piercings and matching check-out jobs, fast become three-dimensional. Even the standard characters you find in most high school movies (the best friend, the mean girl, the guy who’s too cool for school, the hunky teacher) are less cartoonish than they might be. Part of the fun is thinking you know where a scene is going and being pleasantly surprised when it changes course.