Mike Mills’s C’mon C’mon opens with a series of question – big questions – about modern life and the future, which protagonist Johnny (Joaquin Phoenix), a public radio broadcaster, is posing to kids around the country for a piece. The boilerplate interview ends with “what makes you happy,” the biggest question, arguably, and if you’ve seen more than a movie or two, you’ll probably guess that the film which follows will find this man asking some of these questions of himself. Including the last one. Especially the last one.

Mills is the gifted writer and director of Beginners and 20th Century Women, and he again presents us with a portrait of a family far more complicated than the typical, nuclear variety. Johnny is unmarried, and has no children, and has felt particularly untethered since his longtime girlfriend left him and his mother died, around the same time, about a year before the story begins. But he has an adult sister, Viv (Gaby Hoffman), who is no longer living with her biopolar husband Paul (Scoot McNairy), making her the primary caregiver for their nine-year-old son, Jesse (Woody Norman). But Paul is having some trouble, and Viv needs to go lend a hand. “So who’s watching Jesse?” Johnny asks.

And that’s how “Uncle Johnny” ends up comes to L.A. to help out. “He loves it when I’m not around,” Viv assures him. “You’ll have so much fun.” But it’s not always that simple.

So what we have here is basically a 2020s riff on Kramer vs. Kramer, the modern man, unexpectedly pressed into child-rearing, and finding something of himself in the process. Like that film, C’mon C’mon occasionally lapses into too-cute/too-clever territory, but not often; Mills has a keen understanding of how kids’ brains work, the way they create strange little alternate personalities and realities, and an ear for how they communicate those strange connections. He makes the kid “weird” without leaning on lazy quirks of affectations, and Norman, who is quite gifted, deftly captures the big emotions of this age.

Meanwhile, Phoenix and Hoffman do the same for theirs. When Johnny complains to Viv that he’s not yet used to the whole parenting thing, she counters, “I’m not used to it. I fucking hate it.” This prompts a long, candid jag about the difficulties of parenting, a scene that rings with so much truth, it may as well be the Liberty Bell. Mills is a parent, Google assures me, but I didn’t really have to check; in scene after scene, he works from an insider’s understanding of how quickly and entirely helplessness can overwhelm you, when you’re in charge of a child, from the little moments of impatience and frustration to the very specific, earth-shattering terror of losing track of a kid, even for just a moment.

So these two fine actors function as both a study in contrasts – the new “parent” and the long-timer – using this unexpected common ground to reconnect. Hoffman has gone from a marvelous child actor to an uncommonly effective “grown-up”; she conveys all of her complicated and painful history in these loaded interactions, the conversations where she’s saying one thing when she clearly means another. It’s a tough relationship to sell, the adult sibling dynamic, and these actors nail it.

Phoenix gets the showier role, of course – more screen time, but also much more of a journey, as this period of responsibility ultimately requires him to figure himself out. This isn’t mere pageantry; Mills’s insightful script understands how being in charge of a kid means having to more acutely decipher emotions, and thus, to understand your own. He does so via searching, confessional monologues and increasingly candid phone calls with Viv, where he finds himself re-examining even small, offhand decisions (“I made it into a weird joke. Why’d I do that?’). Too much of this introspection can grate, but Phoenix is clearly invested in this character, and his deadpan delivery proves a good comic weapon.

That said, C’mon C’mon plays a bit stronger in retrospect than it does in the moment. The picture meanders -– sometimes charmingly, sometimes indulgently. And the pacing is just a little punchy; it runs 108 minutes but feels longer, and not always in a good way.

But that shagginess pays off by the closing scene, which lands with an emotional wallop. “Nobody knows what they’re doing with these kids,” Viv explains. “You just have to keep doing it.” That’s the kind of truth that makes you forgive a film’s messiness, and though it sounds strange to phrase it so bluntly, C’mon C’mon is a movie that made me want to be a better parent – and a better person. And that’s not nothing.