A manic depressive father must take full responsibility for his two daughters after his ex-wife decides to go back to college. (Sony Pictures Classic)

The manic-depressive father at the center of “Infinitely Polar Bear” isn’t the only thing subject to wild mood swings. More so than even the usual dramatic comedy, the semi- autobiographical movie careers from the sweet to the sour. These shifts occur without warning and in a manner that’s as hard to make sense of as it must have been for writer-director Maya Forbes to comprehend when, as a child, her father was diagnosed with bipolar disorder.

The film’s title comes from a child’s malapropism for the disease, and it’s uttered by Faith, a stand-in for Forbes’s younger sister, China (here played by 9-year-old Ashley Aufderheide). Forbes’s real-life daughter, Imogene Wolodarsky, plays Amelia, a 12-year-old version of the filmmaker. Together, these young actresses are the best things about the movie, giving it whatever sweetness it possesses.

The fun dad/scary dad act that Mark Ruffalo turns in as Cameron Stuart is appropriate, especially as filtered through the lens of memory. Forbes presents her father’s manic depression as something of a roller coaster ride: world’s best stay-at-home dad one minute, making chocolate truffles from scratch, and willful, overgrown child the next, as he abandons the girls to go out drinking. In her telling, neither extreme is particularly threatening.

This view through rose glass is jarring, as Forbes pushes Cam’s manic episodes too far in the direction of wacky escapades — complete with irritatingly jaunty theme music — seemingly underplaying the seriousness of his condition.

As for Ruffalo, underplaying is not the problem. His performance comes across as a bundle of mannerisms. Cam’s a twitchy, chain-smoking goofball whose symptoms suggest the affectations of an eccentric and whose genuine endangerment of his daughters is only rarely acknowledged by the tonally incongruous script. First-time director Forbes is best known as a writer for “The Larry Sanders Show” and, with husband Wallace Wolodarsky, for the screenplay adaptation of “Diary of a Wimpy Kid: Dog Days.”

The premise of “Polar Bear” is that Faith and Amelia’s mother (Zoe Saldana) has gone to business school, as Forbes’s mother did, leaving her children in the care of her estranged and unemployable husband, who gives no evidence that he is capable of taking on such responsibility. A scion of a wealthy New England family, Cam gets no assistance from his grandmother Gaga (Muriel Gould), who controls the purse strings to the family fortune, so the exigency of his financial circumstance is palpable.

Be that as it may, the sugar-coating chronically undermines the film’s attempt at memoir. Forbes certainly didn’t set out to make a documentary, and she needn’t be concerned about her father’s reaction to his on-screen portrayal; Cameron Forbes died in 1998.

But what “Polar Bear” really lacks is hindsight. It is a little girl’s valentine to her father, without the benefit of bittersweet wisdom that comes with age.

R. At area theaters. Contains crude language. 88 minutes.