Maya Forbes' directorial debut goes beyond autobiographical. It is about as personal as a film possibly can be -- the story of her childhood, written by her and tangibly told from the perspective of one of her daughters, cast by her to play a younger version of herself.

It's a story worthy of a feature film to be sure. Forbes is the product of biracial parents. Growing up in Boston in the 1970s, she was raised for years by her white father, a manic depressive, while her black mother studied and worked a city away, earning her MBA and then working in finance in New York.

Infinitely Polar Bear picks up with the dissolution of her parents' marriage. Her father Cam and is dragged away by police at the height of an episode in the film's opening moments. He is not dangerous or violent exactly, but his behavior is scary enough to turn his wife Maggie and his two daughters in to de facto hostages. Just as important as the circumstances of those first few frames tonally is what her father is wearing -- a Speedo that would qualify as modest only in certain parts of the Mediterranean and, taste notwithstanding, is an exceedingly poor wardrobe choice on a dreary, gray New England day.

Right from the very start, Forbes lays bare a childhood wound -- a dark, sad memory of having a father with a serious mental illness -- and yet she also manages to find a sliver of humor. And so goes the rest of the film, running the full gamut of emotions from moment to moment and scene to scene -- joy, love, sadness, poverty, education, parental embarrassment, abandonment, fright, belonging, identity and so forth.

The central tension stems from the excruciating decision made by Forbes’ mother Maggie, portrayed by Zoe Saldana, to not just leave her young daughters behind in another city so she can better provide for them, but to leave them with her unstable husband Cam, portrayed by Mark Ruffalo, mainly because she has no other choice. He is determined to win his wife back — and do it by showing he can care for their daughters — but his demons, often egged on by self-medication in alcoholic form, are waiting around every corner.

Forbes’ characters are rich — how could they not be in this case — but perhaps a little bit too rich for such a brisk film. Her mother is courageous yet fragile. Her father, born to old Boston money, often flashes the demeanor and bluster of privilege but is paradoxically deeply insecure. He can go from totally selfish to absurdly selfless in a matter of moments or even do both at once, most notably when he sets out to make his younger daughter a dress and stays up all night to complete it, drinking heavily the whole time. The two girls at the heart of the action, played by her daughter Imogene Wolodarsky and Ashley Aufderheide, are precocious, but not unbelievably so considering how symbiotic their relationship with their father is. They are caring for and protecting him just as much as he is for them, making their more adult moments believable as a result.

Though her characters prove themselves worthy of affection over and over, Forbes struggles with her own story. She touches on a great deal — all those emotions I listed a few paragraphs ago — but never really settles on a theme (or two or three) that her viewers can easily take out of the theater. This might make her story more “true,” but it also makes it difficult to consume, frequently draining it of narrative momentum. You could argue that this unevenness is designed to heighten the sensation of being parented by a manic depressive, but (having attended a Q&A session with her) I don’t think Forbes would make that case herself, and even if she did, I think that would be cutting her entirely too much slack.

So what you are left with is half of a very good film — lovable characters played admirably by the likes of Ruffalo, Saldana, Wolodarsky and Aufderheide acting out a story that is too ambitious to be fully realized over the course of 90 minutes. I am eager to see more from Forbes, but not to revisit her first effort.