No surprise writer-director Mike Mills’ “20th Century Women” is semi-autobiographical, given the authentic, bittersweet, sometimes lyrical, nostalgia-baked tone of the wonderful screenplay.

The real surprise is the source material wasn’t a novel. Even as a movie, “20th Century Women” feels like a slim but engrossing novel you read twice in college and tell all your friends they have to read it as well.

Set in Santa Barbara, California, in the late 1970s, “20th Century Women” is told from the point of view of 15-year-old Jamie (Lucas Jade Zumann), who is growing up in a boarding house that appears to be in a constant state of renovation.

Jamie’s a typical teenage boy — a little bit lost, a little bit adventurous, a little bit horny, a little bit scared. His journey is the least interesting element in the story, but that feels at least partially by design, as his main purpose is to serve as a conduit to tell us about three unique, colorful, deeply influential women who will shape his life forever:

• Abbie (Greta Gerwig), punk-loving aspiring photographer in her 20s with a crazy hairstyle and an almost manic thirst for embracing life.

• Julie (Elle Fanning), a neighbor girl who is two years older than Jamie and is sexually active with a number of boys — but when she climbs into bed with Jamie nearly every night, she makes it clear things must remain platonic, because their friendship is something special and sex will only ruin it.

• And most prominently, there’s Jamie’s mother Dorothea (Annette Bening), who was not young when she gave birth to Jamie, found herself raising Jamie by herself after a divorce — and is feeling increasingly lost in her efforts to connect with her teenage son.

Dorothea asks Julie and Abbie to in effect help her raise Jamie. Given that Abbie lives in the house as a boarder and Julie is at the house nearly every night, it kinda feels like they’re already doing that anyway in their own fashion, so it’s no big stretch for them to agree.

Another fixture in the house is William (Billy Crudup), a carpenter and all-around handyman who is spearheading the renovations. (Crudup is so good playing easy charmers who are well aware of their effect on women and not averse to taking advantage of it — but feel almost sheepish doing so.)

Dorothea is a fascinatingly complex woman. When her old beater of a car catches on fire in a parking lot and Dorothea invites a couple of firemen for dinner that night, she comes across as nearly pathetic — until we cut to the dinner and we realize the firefighters are fortunate to have been invited. And so it goes for much of the journey, as Dorothea becomes an ever-more interesting character, thanks to Mills’ writing and Annette Bening’s beautifully calibrated performance.

At times the late 1970s cultural touchstones seem a little forced and overdone, what with Abbie reading “Our Bodies, Ourselves”; Dorothea trying to understand punk rock, and a group viewing of President Jimmy Carter’s infamous “malaise” speech, in which he spoke of a “crisis of confidence” among the American people. And some of Jamie’s exploits are dumb and selfish, even for a 15-year-old, making us impatient for the focus to shift away from him and back to the woman or even William the handyman, all of who are more fully drawn and compelling figures.

Crudup, Gerwig and Fanning are superb — but this is Annette Bening’s movie, and she delivers one of the finest performances of her career as a middle-aged hippie mom who refuses to let her own disappointment and heartbreak get in the way of fighting with every fiber of her being to make sure her son has the best life he can possibly have.

There’s not a single false, “actor-y” note in Bening’s work. It is a master class in nuanced acting, and it is deserving of an Academy Award.

“20th Century Women” co-star Lucas Jade Zumann will participate in a Q&A after the film’s 7:30 p.m. Saturday screening at Landmark Century Centre.

★★★

A24 presents a film written and directed by Mike Mills. Rated R (for sexual material, language, some nudity and brief drug use). Running time: 118 minutes. Opens Friday at AMC River East, Landmark Century Centre and Century 12 in Evanston.