To watch Infinitely Polar Bear is to marvel at the talent of Mark Ruffalo, an actor who appears equally at home in drama (The Normal Heart) and action (Avengers: Age of Ultron), and whose star is clearly on the rise. He’s been good before, as Oscar nominations for Foxcatcher and The Kids Are All Right suggest, but he’s never been better than he is in director Maya Forbes’ drama Infinitely Polar Bear.

In the film, Ruffalo plays Cam Stuart, a whip-smart manic-depressive who’s as unpredictable as he is loving. The setting is 1978, and his wife Maggie (Zoe Saldana) loves him but is concerned about his erratic behavior (for example, chasing a car wearing only his red underwear in the dead of winter). And so it’s with great trepidation that Maggie leaves their daughters Amelia (Imogene Wolodarsky) and Faith (Ashley Aufderheide) behind with him in Cambridge, Mass., so she can attend graduate school in New York City. Cam, with a full bottle of lithium medication in the cabinet and daunted by the task of caring for two pre-teen girls for 18 months (Maggie visits most weekends), accepts his newfound responsibility with open arms and little clue of what’s in store for him.

What’s great about Ruffalo’s performance is that Cam is a caring, devoted father who happens to be manic-depressive, not a manic-depressive who tries to be a good father. This is absolutely key, because we never doubt his love for his wife and children, only his ability to care for them. If the story were more focused on the disease, we’d see him in doctors’ appointments and more emphasis would be given to his treatment, which would be less interesting and minimize the emotional impact. But watching Cam handle two sprightly young girls (both Wolodarsky and Aufderheide make their film debuts here, and they’re wonderful) feels organic and unforced, a man out of his element and plagued by his own mind but always doing his best for his daughters.

In fact, sometimes he does too much. A running joke comes in the form of Cam’s repeated attempts to be a good neighbor in their apartment complex. He doesn’t grasp the social conventions involved: It’s nice to carry a woman’s (H. Tod Randolph) grocery bags to her unit, but then offering help putting groceries away and peeling onions for dinner is too much, and horribly embarrassing for his daughters. If only that were the worst of his conduct. Other questionable moments range from smoking too much to not getting the girls up in time for school to leaving them behind to go out drinking.

Forbes, who also wrote Infinitely Polar Bear, based Cam on her own father, and the story comes from her experiences as a girl. How much is true and how much is artistic license only the writer-director knows, but it doesn’t really matter because everything that happens serves the story well.

Impressively, Forbes does not succumb to the shortcomings that plague other first-time directors (bloated story, too many characters, pacing issues, etc.), and keeps the film moving at a brisk pace for an 88-minute run time that’s neither too long nor too short. There are no plot holes, no gaps of logic, and no scenes that feel tedious or unwanted. Everything is nicely explained, you care about the good people involved as they navigate this tumultuous time, and at no point is it maudlin or overly sentimental.

Thank you, Maya Forbes, for understanding exactly what your movie should be and executing that vision extremely well.

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