Written and directed by Christy Hall, the terribly titled “Daddio” is built upon the simplest of concepts – two people in a car talking. That’s the movie from start to finish. It’s a self-imposed limitation that keeps things intimate and focused while allowing the two stars (Dakota Johnson and Sean Penn) to be the centerpieces. At the same time, it’s a limitation that demands a sharp and engaging script that keeps the audience locked in and involved. And that’s where “Daddio” sometimes struggles.

Johnson plays a young woman who arrives at JFK International Airport after a late-night flight home from Oklahoma. She hops in a cab with a driver named Clark (Penn) who sets out for her apartment in Midtown. As they travel through the New York City night, the shamelessly chatty Clark opens up a series of conversations that go from paltry and crass to intimate and personal. And that’s “Daddio” in a nutshell.

As you would expect, we learn a lot about the young woman and the cabbie over the course of their drive. We learn the crude and oblivious Clark has no filter and doesn’t mind saying or asking anything that comes to mind. We learn he has driven a cab for 20 years which apparently has turned him into some kind of gritty, tough-minded sage. Over that time he has formed a variety of pessimistic (and frankly warped) views on life, love, and being a man. And he’s not afraid to share them (as we quickly discover).

As for the young woman, she’s a computer programmer returning from a trip to see her half-sister. A lot of pent-up pain comes to light that’s directly related to her family history. Much of her story comes out through her boyfriend who we mostly get to know through a series of lewd and perverted texts that she receives throughout the cab ride home. And the more she and Clark open up to each other, the heavier her revelations get.

“Daddio” isn’t built upon the most original idea. But strong, lived-in performances from Johnson and Penn make this a mostly competent two-hander. Still, they can only do so much, and the film’s ultimate undoing is the script. Aside from essentially going nowhere, there’s a level of implausibility with it that’s hard to get past.

For example, call me skeptical, but would a broad probing conversation like this ever really happen in a taxi? Who knows – maybe there are professional cabbies who also moonlight as deep-thinking psychologists and philosophers. Even more out there, I find it hard to imagine that any woman would willingly stay in the car with a man this bawdy and invasive. Perhaps I’m too skeptical, but these things are dubious enough to make us question what we’re seeing.

“Daddio” may be worth seeing for the performances alone. Johnson only gets better with each new role (we will pretend “Madame Webb” never happened), and Penn has long shown a confidence and sturdiness when given the right material. But Hall has a hard time maintaining a steady sense of drama and she stretches her story out for a little too long. She smartly leans on her two more than capable stars, but doesn’t always give them what the need to make her movie click. “Daddio” is now playing in select theaters.

VERDICT – 2 STARS