There’s a bit of an acting battle going on in “Boundaries.” On the one side is Vera Farmiga as Laura, a Seattle single mother struggling to raise a teen son, Henry (Lewis MacDougall), while taking in every stray dog and cat that crosses her path. On the other is Christopher Plummer as her ne’er-do-well father Jack, whose intermittent intrusions into her life have sent her into psychotherapy. Farmiga acts up a storm, while Plummer offers wry understatement in response. Guess who wins.

As is often the case, experience tells. Farmiga, trying much too hard, comes off as shrill, while Plummer is mostly content to look on bemusedly and deliver his lines with a sneaky smile, knowing that he’s walking off with the movie without even working up a sweat. Of course, at 88 he has had plenty of time to learn the noble art of scene-stealing.

Unfortunately, in this instance there isn’t much to pilfer. Shana Feste has made one of those dysfunctional family road movies that assemble quirks like well-spaced speed bumps on the pavement before reaching an entirely predictable destination. Even Plummer’s savvy can’t salvage the insipidity.

Laura (Farmiga) is continually frazzled trying to keep her messy household running, filled as it is with animals and an occasional male visitor none too happy with the environment. Her major concern, however, is with Henry, a kid who simply doesn’t fit in at his local school. In fact, he’s just been expelled not so much for getting into fights (though he does), as for specializing in drawings that depict their subjects naked in poses that, he says, “show their souls.”

The principal recommends private schools that would be more suited to the boy’s needs, but they’re expensive, and after being rebuffed by her extremely rich but loopy boss, Laura has no other option but Jack. He’s just been tossed out of his retirement community for growing marijuana in a gardening shed, and needs a place to stay. He promises to cover Henry’s school costs if Laura provides him with new living arrangements. She refuses to take him in herself, but convinces her sister JoJo (Kristen Schaal), a doofus dog-walker in Los Angeles, to share her apartment with him.

Jack agrees, but insists that Laura drive him to L.A. in his old car, overcoming her resistance by somberly telling her that his cancer has returned and he wants to spend time with her. That sets up the road trip, with Henry in the back seat.

But Jack has an ulterior motive for the long ride: he’ll be selling a couple hundred thousand dollars of weed to buyers along the way, enlisting Henry as his “partner” as a means of bringing the kid out of his shell. There will be stops at a community of Buddhist monks and a hippie commune, but the most colorful customers are an art forger (Christopher Lloyd), who serves the family a candy corn casserole, and a long-time buddy (Peter Fonda) who has to fend off a couple of inept housebreakers during their visit. Both actors engage in genial routines with Plummer that wouldn’t have been out of place on the vaudeville stage.

There’s one other stop, to visit Laura’s ex-husband Leonard (Bobby Cannavale), which Jack obviously intends as a lesson to both Laura and Henry that they should have no illusions about the guy.

By the time the trio gets to L.A., after many mishaps and shouting matches, Jack and Henry have bonded, and even Laura has begun to mellow in her attitude toward disreputable dad. But mother and son go home alone—though you know full well they won’t stay that way, and we’re not talking about their menagerie of rescue animals.

Through it all Plummer sails along without a care, making it all look effortless while Farmiga huffs and puffs mercilessly. McDougall brings a stealthy smirk to the proceedings but otherwise is rather bland, while Cannavale plays a louse convincingly and Schaal does the charming goofball routine with her usual aplomb. Lloyd is nutty and Fonda agreeably laid-back. (Fonda’s recent Twitter troubles have led to a call, especially by Trump supporters, to boycott “Boundaries,” but his fleeting presence here makes such extreme action seem pretty pointless. There are more fundamental reasons for skipping the movie.) The technical side of things is okay, though unexceptional.

“Boundaries” is pretty much paint-by-numbers filmmaking, but Plummer makes it less painful than it would otherwise be. He’s worth watching, even if the movie doesn’t do him justice.