Infinitely Polar Bear really shouldn’t work.

The story of a manic-depressive father raising two girls in the late 1970s looks at first glance like a mawkish depiction of mental illness, a twee domestic fairytale that smooths over the rough edges of its subject. To wit: In an early scene, the father, Cam (Mark Ruffalo), rides a bicycle in a Speedo in the middle of winter. And even when he goes dark, it’s not quite as serious as we expect—he drinks and smokes, but causes no harm to himself or his kids.

But the key to appreciating Infinitely Polar Bear is understanding that it’s not an issue movie; it’s a memoir. Writer-director Maya Forbes based the film on her own upbringing with a bipolar father in Boston. As such, we see the story through the eyes of the children, Amelia and Faith, the latter of whom coins the film’s title (she can’t remember the term “bipolar”). After their father’s first mental breakdown, his wife Maggie (a winning Zoe Saldana) realizes the family cannot live on her low salary, so she hatches a plan: She’ll go to New York, get her business degree in a year and a half, and allow Cam, fresh out of a halfway house, to take care of the kids.

It’s a high-stakes situation, as there’s nothing to indicate that Cam is ready for the responsibility. He goes out drinking and leaves the kids home alone. He waltzes them into a stranger’s home and asks for a tour under the dubious proposition that his great-grandmother was born there. When the kids push him, he reacts like a child himself, screaming, cursing, and throwing things. (In one scene, he drops them off at school after an argument and screams out the window, “Have a nice fucking day!”) Forbes gets the tone just right: The opening scenes are home movies of the family during the good times, before his breakdown, and while the film eventually abandons that formal technique, its impact lasts. Infinitely Polar Bear never feels like anything less than the smoothed-over memories we all have of our own childhoods.

Often considered a thinking man’s movie star, Ruffalo gives an electric performance in sync with Forbes’ mission, painting a convincing portrait of a brain on overdrive. Given the film’s rosy tone, Cam spends more screentime than not as a playful eccentric, and Ruffalo appears to be having way more fun than he ever could as the Hulk.

It’s an astonishingly risky approach by both Forbes and Ruffalo that leaves them open to criticism from groups rightly concerned with accurate depictions of mental illness. Still, there’s something brave about taking a dark, painful subject matter and choosing, matter-of-factly, to be whimsical instead of morose. Of course, it helps that it’s authentic: Anyone who’s ever had a relationship with a person suffering from mental illness knows that it’s not all doom and gloom. It can be painful, and it can be joyful, and often both at once. Infinitely Polar Bear gets this right, and it’s enough to make anything it gets wrong seem unimportant.

Infinitely Polar Bear opens June 26 at E Street Cinema.