It’s too easy to say that with his new film Lawrence Kasdan, once among the most influential of our younger crop of writer-directors (starting his career with the twin touchstones of “Body Heat” and “The Big Chill” in the early eighties), has gone to the dogs. After all, his work over the last thirty years has been a relentless downhill slog, a series of missteps only occasionally lightened by flashes of the old promise. But “Darling Companion” is a shaggy-dog tale that, despite a talented cast, is a cinematic mutt.

The anchor of the ensemble script is a well-off upper-class Colorado couple, back surgeon Joseph Winter (Kevin Kline) and his wife Beth (Diane Keaton). As the picture begins, she’s seeing off their married daughter and grandchild, and on the way home from the airport with her younger daughter Grace (Elisabeth Moss) the two women encounter an abandoned dog on the side of the snowy road. On impulse they rescue it, have it looked at by a likable vet Sam (Jay Ali) and adopt the animal, much to self-absorbed Joseph’s discomfort.

Months later the Winters are celebrating Grace and Sam’s wedding at their rustic vacation home in the Rockies. Staying behind from the party are Joseph’s sister Penny (Dianne Wiest), her new boyfriend, buffoonish Midwesterner Russell (Richard Jenkins), and her son Bryan (Mark Duplass), who’s also a spine surgeon. Penny and Russell, both recently retired, intend to open an English pub in Omaha—a prospect to which both Joseph and Bryan react with dismay. Also on hand is the Winters’ darkly beautiful Carmen (Ayelet Zurer).

What passes for plot kicks in when the dog, named Freeway, runs off into the woods while on a walk with Joseph. Beth erupts, obviously having made the mutt her surrogate child. Her over-the-top response bewilders her unflappable husband, who’s been too dense to realize that his wife has long felt abandoned and unappreciated. Before long they and all their houseguests are tramping around the wilderness trails in pursuit of the canine, eventually taking their cues from Carmen, who claims to possess psychic powers. Their wanderings lead to encounters with some oddball locals, including a red-haired lady with a mutt named Blue, a Unibomber-like recluse and the sheriff, a gruff fellow with kidney stones played by Sam Shepard with good-old-boy charm.

Ultimately, though, the movie isn’t about the search for the dog as much as the human relationships that develop and change over its course. Joseph and Beth find what they’d lost over the years, Bryan and Russell learn to like and respect one another, and Penny and Russell grow ever closer, and Bryan and Carmen fall in love. All of this is portrayed through ultra-cute quietly revelatory episodes, dialogue by Kasdan and his wife Meg that includes way too many obvious soundbites (often sounding like excerpts from a droopy sitcom), and performances that, while genial enough, lack energy and snap. As a whole the picture just stumbles along, much like the characters in the woods, before culminating in a final burst of comic action aboard a commuter plane to Denver that, in these days of extreme airline security, leaves a rather sour taste.

One doesn’t want to be too hard on “Darling Companion”—that would be too much like kicking some mangy mongrel that’s overly anxious to please. Nor would it be kind to point out that technically the picture is a pretty scruffy affair; the outdoor demands were apparently a bit too much for cinematographer Michael McDonough, whose images are often a mite murky (when they’re not simply bland). It’s better just to allow this sadly undernourished effort from a filmmaker who once held such promise to expire as quietly and humanely as possible.