Way too many films out there are heavy on plot and exposition, light on atmosphere and character development. It's safe to say "20th Century Women" isn't one of them.

Indeed, this new film from Mike Mills, a loving nod to the director's own 1970s California youth, is precisely the opposite: a thoughtful and detailed evocation of an era and especially of one complicated character, with little story to speak of.

Such atmospheric films, however expertly done, can either charm you or frustrate you to pieces. Luckily, even if it's the latter, Annette Bening is there to pick those pieces up.

There should be no doubt by now that Bening, at 58 a four-time Oscar nominee, is one of our finest actresses. Her recent turn in husband Warren Beatty's "Rules Don't Apply" was brief, yet delightful. That was a mere cameo compared to her passionate and meticulous work in "20th Century Women," a movie that truly revolves around her.

If Mills' 2010 "Beginners," which won Christopher Plummer an Oscar, was about his father, "20th Century Women" - also semi-autobiographical - feels like a companion piece about his mother. Bening is Dorothea Fields, a 55-year-old single mom of a 15-year-old boy, Jamie (Lucas Jade Zumann, in a sensitive and unmannered performance). The time is 1979, the place Santa Barbara. The world is brimming with change, both cultural and political.

Like the time in which she lives, Dorothea is hard to figure out - a mess of appealing contradictions. She is free yet guarded, proud yet insecure, forceful yet tentative, cynical yet naive. A few constants: She pays strict daily attention to the stock tables. She wears Birkenstocks. She chainsmokes Salems - her excuse is that when she started, smoking was simply stylish, not bad for you.

Most obviously and urgently, Dorothea loves her son, and is trying to figure out how to raise him as a "good man" in a changing world.

Hers is not a traditional parenting style. When Jamie is called to the principal's office for missing school, Dorothea earnestly wonders why he shouldn't be able to skip school if he wants. She's told that he needs a good excuse. And so, Mom starts sending notes like: "Jamie was doing volunteer work for the Sandinistas."

Yet despite her unconventional approach, Dorothea faces struggles familiar to all parents, such as a teen's blind feelings of invulnerability. One day, Jamie joins friends in a game that involves briefly falling unconscious. Except, Jamie doesn't wake up. He ends up in the emergency room. Dorothea's raw anguish as she asks her son the million-dollar question to which no teen has an answer - "What were you thinking?" - is agonizing to behold.

With no partner to co-parent Jamie (Dad's out of the picture), Dorothea concludes it will take a village. She turns to two other women in Jamie's orbit.

First is Abbie (Greta Gerwig, absorbing to watch as ever), a 24-year-old punk artist renting a room in Dorothea's rambling house. Abbie's hair is dyed a brilliant shade of fuschia, a la David Bowie. She, too, is a free spirit, but her life also has a darker, more poignant side: she's a survivor of cervical cancer, and has been told she won't be able to have children.

Then there's Julie (Elle Fanning), only two years older than Jamie. This beautiful blonde creature is Jamie's best friend, but also his tormentor: She sneaks into the house every night to sleep in his bed, but refuses to become sexually involved with him. This is, needless to say, quite frustrating for an adolescent boy.

Rounding out this unusual group is William (Billy Crudup), a handyman who is (slowly) renovating Dorothea's ramshackle abode. The only adult male character of substance, William serves a different purpose for each female in the film.

He has a terrific seduction scene with Abbie - painfully awkward yet still romantic. She compliments how his hair smells; he replies that he makes his own shampoo. But Crudup's most entertaining moment is with Bening, when William tries to help Dorothea figure out the new music the kids are listening to these days, in a sort of Black Flag versus Talking Heads danceoff.

Somewhere in the middle of this absorbing and unabashedly meandering film, one comes to better understand the title. The three women, all born in different eras of the 20th century, each have something to teach young Jamie.

And Jamie is coming of age at a time when things are changing for women, too. Abbie gives Jamie some feminist texts to read, from which he learns, among other things, about the nature of the female orgasm. In one of the funnier episodes, he tries to educate a fellow teenage boy, with predictably disastrous results.

"20th Century Women" is narrated by its young protagonist and as such, feels like a coming-of-age story. But whose? The story really feels like Dorothea's. Certainly hers is the most fully realized character and the most interesting by a mile - even if at the end, we're not quite sure what she's learned, what it's all added up to.

But no matter: that often bemused look on Bening's face seems to be telling us that it's really the journey that counts.