A low-key, unassuming but pleasant family road dramedy, “The Discoverers” features a rare lead performance by Griffin Dunne, who’s been relegated to secondary roles for so long that one might have come to doubt his ability to carry a picture anymore. He shows he hasn’t lost his touch.

Dunne plays Lewis Birch, once a promising young history professor at Yale whose career has gone as far south as his marriage. Now teaching at an unaccredited community college where his high expectations of students might lose him even his adjunct job, he’s still laboring on his book, a study of York, the slave on the Lewis and Clark expedition to the Pacific in 1804-06, that has already reached 1600 pages in manuscript. He’s going to a conference in August to deliver a paper promoting the planned publication of the book by a small university press, but first must pick up his teen children Zoe (Madeleine Martin) and Jack (Devon Graye) from their mother’s place in Chicago to drive there with him. They’re not exactly enthusiastic about the trip.

To further complicate matters, along the way Lewis gets a call from his brother Bill (John C. McGinley), informing him that their mother is gravely ill at the family’s Idaho home. Since he’s been estranged from his parents for years, he’s reluctant to change routes to visit her, but relents. When he arrives, however, he finds his mother dead and his father Stanley (Stuart Margolin) rendered speechless and sullen at the loss.

Lewis plans on leaving for the conference as soon as the funeral is over, but before he and the kids can get underway, Stanley disappears. He’s gone off to participate in the annual reenactment of the Lewis and Clark expedition that was always a family tradition, and on the advice of Stanley’s doctor Lewis follows him to the gathering, where Stanley has assumed the part of the imperious Clark. Soon he, Zoe and Jack have become reluctant members of the group, dressed in an approximation of nineteenth-century clothes and forced by Cyrus Marshall (David Rasche), who’s assumed the role of Meriwether Lewis, to give up their non-conforming modern gadgets for the duration (though that proves an unenforceable restriction).

The journey by foot westward will, of course, bring folks together. Lewis strikes up a friendship with fellow traveler Nell (Cara Buono), and Jack an increasingly physical one with Cyrus’ buxom daughter Abigail (Dreama Walker). More importantly, Lewis bonds with Zoe, a perceptive, quick-witted girl who can teach her father as well as learn from him. And though there are significant road bumps along the way, Stanley will eventually break out of his protective shell, abandon the unpleasant persona of Clark and reconcile with his son.

“The Discoverers” has elements that are never fully dealt with. Lewis’ work on York is obviously meant to mirror his own enslavement to a past he’s never come to terms with, for instance, but that’s a connection that’s never really fleshed out. Nor is the reason behind the hostility between Lewis and Stanley ever made clear.

Still, the relaxed, unforced atmosphere fostered by writer-director Justin Schwarz and cinematographer Christopher Blauvelt’s homely but evocative use of the settings, especially in the outdoors, give the performers an opportunity to shine. Dunne makes a likably laid-back protagonist, and gets an opportunity to show off his skill at acting out in desperation when Lewis learns he’s been beaten to the punch about York by a rival writer he detests. Martin paints a nifty portrait of a teen without illusions, and while Graye is more anonymous, Rasche, hidden behind a beard, offers a funny turn that’s also one of his more restrained. Both Buono and Walker are attractive presences, though neither has much heavy lifting to do. The only cast member who comes off less than persuasively is Margolin, but that’s more the fault of the writing, which makes Stanley an unsettled character prone to sudden and arbitrary shifts of behavior, than of the actor.

This modest but engaging film, which dates back to 2012 and is being released only after a couple of years on the shelf, may be a minor discovery, but it’s a diverting one.