If ever there was a play about the sentimental side of Maine summers, it’s “On Golden Pond,” which opened this week at Acadia Repertory Theatre in Somesville. Written in the 1970s and popularized in the 1980s by a hit film version (starring Henry Fonda and Katharine Hepburn, both of whom won Oscars for their performances), “On Golden Pond” is big on loons, fishing, berry picking, Yankee defiance and local yokels. It is not, however, big on plot.

Nevertheless, Ernest Thompson won the hearts of moviegoers (as well as an Academy Award for his screenplay) with this story of Ethel and Norman Thayer, two oldsters returning for their 48th and presumably last summer in a cabin on a fictional lake in Maine. Their wounded-by-youth-and-carrying-a-grudge daughter Chelsea shows up gingerly for Norman’s 80th birthday. She brings her new dentist boyfriend and his teen son Billy, whom she deposits in the care of her parents while she and the dentist take a European vacation.

The rest is a panoply of one-liners and hand slaps while the young and the old get hip to each other’s groove. You’ll recall that this is the script which dropped the term “suck face” into the language, and, one might guess, upped the annual count of tourists to lakes in Maine.

At the heart of it all is a nicely Disneyesque plot device. Norman learns jive talk. Billy learns about literature and fishing. Ethel scolds them for their boy habits. Together they form an invincibly friendly team. Get it? The boy teaches the man who teaches the boy and both of them get into mischief with the mom?

This is an unfortunate circumstance for the players at Acadia Rep. Director Wayne Loui could go a lot further with folksiness, but doesn’t, bless him. The question is: How do you get still youthful actors to play older than their roles and also keep the action lively without the whole thing looking silly? On opening night, there was laughter and warm fuzzies, but the action could also be belabored and flat.

Ken Stack as Norman, the only fully drawn character in the play, is ornery and, in some crochety way, very entertaining. This is, essentially, his story, which means Alison Cox as Ethel spends a good deal of her time time dusting the table, straightening the cabin, waving her hands with disgust, and waiting for Norman “the old poop” to give her purpose. Cox’s delivery is slow and stilted — and it’s no wonder. As with the role of Chelsea, played valiantly by Catherine LeClair, the part is painfully underwritten.

Alan Gallant as the boyfriend finds some gumption in his precious few minutes onstage, and Zach Robbins as 13-year-old Billy gives an enthusiastic and warm read of the boy.

David Blais, as the bachelor postman who never fell out of love with Chelsea, brings real spark to an otherwise geriatric pace.

It was only after the movie’s success that the play started showing up mirthfully in regional theaters as a popular meditation on summer sunshine and — in case you missed this metaphor — on winter shadows. While the vision of a triumphant married couple growing gray and facing death together is tender, sad and terribly important in terms of themes, the play itself is a Hallmark card. Harmless, of course, but perhaps a bit too cunnin’ for some tastes.

Acadia Repertory Theatre will present “On Golden Pond,” 8:15 p.m. through July 22, and 2 p.m. July 23 at the Masonic Hall in Somesville. For tickets, call 244-7260.