In movies, true-life family stories tend to come so drenched in despair or smothered in treacle that it’s a surprise anyone gets out intact. In her sweet, somewhat nutty feature debut, "Infinitely Polar Bear," writer-director Maya Forbes looks back on her 1970s childhood in a fictional re-creation that, as it brightly skips along, can make you feel like Scrooge for thinking it’s a bit of a snow job. That’s especially the case because the father here, Cam Stuart (the infinitely warm Mark Ruffalo), is an endearingly down-and-out eccentric with manic depression.

‘INFINITELY POLAR BEAR’ Rated: R Opens Friday at Kahala 8

Cam is wearing tiny red briefs and not much else when he makes his big entrance, tearing toward his family on a bicycle in the middle of what appears to be a full-blown manic episode. At that early point in the story, his eldest, Amelia (Imogene Wolodarsky), has already more or less warned you about Cam, having sketched out his past in one of those breezy family-album introductions that turn lives into highlight reels: Harvard, diagnosis, marriage and children. In voice-over, Amelia, who’s around 10, insists that her mother, Maggie (Zoe Saldana), "didn’t care" about Cam’s illness and that the family was happy even if "there’s more to it than that." A few beats later, Cam is coming on like a storm in those red shorts while Maggie is hustling suitcases and children into a car.

The frightened look on Maggie’s face suggests that there’s much more to her and Cam’s story, though we never learn if theirs is a soul connection, an erotic attraction or a delusion. Whatever the reason, she sticks with Cam although he can’t hold down a job and she wants to send Amelia and their youngest, Faith (Ashley Aufderheide), to private schools.

Maggie goes off to Columbia University to earn an MBA, parking her family in Cambridge, Mass.

In many ways, Maggie is the most interesting character in the movie, partly because of her ambition, partly because of how she navigates race. But she’s also underwritten, and the grit that she initially exhibits fades amid the laughter and mothering.

Forbes’ bouncy approach can seem wildly off-key as the screaming fights, questionable parenting choices and sad days pile up. The movie isn’t altogether believable in some of its details or characterizations, perhaps because Forbes remembers it this way or simply likes accentuating the positive.

Maggie strains credibility; Cam playfully demolishes it. He’s about the cuddliest, most adorable mentally ill character imaginable. That’s in keeping with the rest of this movie, which remains insistently upbeat, so much so that it, too, can feel in the grip of a manic episode. It’s an approach that wouldn’t work in a film that engages with mental illness as a cause. But Forbes hasn’t made a movie about her father’s illness; she’s made one about her father, who, through hard and weird times, clearly helped give her what she needed so that one day she could tell this story.