To watch Infinitely Polar Bear is to marvel at the talent of Mark Ruffalo, an actor who appears 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 here.

Ruffalo plays Cam Stuart, a highly intelligent manic-depressive who’s as unpredictable as he is loving. It’s 1978, and his wife Maggie (Zoe Saldana) loves him but is rightfully concerned about his erratic behavior, such as chasing a car wearing only 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 Cam in Cambridge, Massachusetts, 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 preteen girls for 18 months (Maggie visits most weekends), accepts his new responsibility with open arms and few clues.

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 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 have minimal emotional impact. But watching Cam handle two active 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 girls.

In fact, sometimes he does too much. A running joke is Cam’s attempt 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) groceries to her unit, but then offering to help put groceries away and peel 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. Writer-director Maya Forbes based Cam on her own father; the story comes from her girlhood experiences. What’s true and what’s artistic license only Forbes knows — but it doesn’t really matter because everything that happens serves the story well.

It’s impressive that Forbes doesn’t 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, and we care about the good people involved as they navigate this tumultuous time. 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.