The coming-of-age drama “So B. It” is an odd duck, a mix of 12-year-old quirky optimism and grown-up anxieties that never come together in a complete story.

This adaptation of Sarah Weeks’ young-adult novel centers on Heidi It (played by Talitha Bateman, last seen in “Annabelle: Creation”), a 12-year-old living in Reno, Nev., with her autistic mother, So (Jessica Collins). So paints childlike pictures, speaks only 22 words and is incapable of caring for herself. Heidi and So live with a guardian, Bernadette (Alfre Woodard), whose acute agoraphobia keeps her from leaving their shared apartments without debilitating panic attacks.

Heidi has a gift for luck, which she and Bernadette sometimes use to make ends meet. She’ll dress like an adult, drop a quarter in a slot machine and rake in the winnings to pay for, as an example, a new part for the vacuum cleaner.

Once, when this happens, it sparks in Heidi a determination to learn more about her mother’s past. She finds a roll of film, has it developed and finds pictures of her mother when she was pregnant with Heidi — living in what looks like a group home, somewhere in upstate New York. Heidi hits a slot machine, pays for a bus ticket and gets a kindly grandmother type (Cloris Leachman) to act as her guardian to get past the bus driver.

Up to this point, the tone created by director Stephen Gyllenhaal (a veteran of TV and movies, but now better known as Jake and Maggie’s dad) is a light family-friendly adventure. Once on the road, though, things get dark and disturbing, particularly when Heidi arrives in the town of Liberty, N.Y., and encounters the founder of her mother’s old group home (played by John Heard, who died in July).

“So B. It” boasts solid performances from young Bateman, Jacinda Barrett as a kindly nurse, and Woodard, who is congenitally incapable of bad acting. But they are working in service to a story, adapted by screenwriter Garry Williams, that shifts unsteadily from preposterous to heartbreaking without feeling authentic at any moment.

* * 1/2

So B. It

A 12-year-old girl seeks the truth about her autistic mother’s past in this drama hobbled by massive mood shifts.

Where • Megaplex Jordan Commons (Sandy) and Megaplex 20 at The District (South Jordan).

When • Opens Friday, Oct. 13.

Rating • PG-13 for some thematic elements.

Running time • 98 minutes.