Overheard as the credits rolled for Poms: “Diane Keaton can do no wrong.”

That may not be entirely true, but the divine Ms. K is nearly always a joy to watch, and she keeps this senior comedy from veering off into inanity or sentimentality more than once.

Keaton plays Martha, a single, childless woman who starts the movie by telling an unseen healthcare worker that she’s refusing chemo. Then she sells her belongings, leaves the homey New York apartment she’s lived in for 40 years and moves into a sterile house in a Georgia retirement community. “I’m just here to die,” she tells the bossy welcoming committee led by Vicki (Celia Weston, TV’s Modern Family). They think she’s kidding; we know she’s not, and the silently ticking clock of Martha’s illness gives the movie a certain urgency.

It needs that undertow, because what’s on the surface is remarkably shallow. Retirement communities ought to be ripe sources of comedy and melodrama, but Poms sticks with a simple Mean Girls vibe: Martha and her spunky neighbor, Cheryl (Jacki Weaver, Widows), start a cheerleading club; sour, bossy Vicki does her best to shut them down.

The screenplay, by newcomer Shane Atkinson and director Zara Hayes (Poms is her first narrative feature), is a catalog of missed opportunities. The talent-rich cast of elderly cheerleaders — including Rhea Perlman, Pam Grier and the wonderful Phyllis Somerville — are mostly given one-joke back stories, and there’s no attempt to sketch in any of the realities of retirement living.

It’s like the writers never actually visited such a community; any Asheville resident could tell ten more amusing senior life anecdotes just from shopping along Hendersonville Road.

Which brings us back to Keaton, who somehow takes a nothing character and fills her with emotion and longing, a magic she shares with her Poms squad, who are much more charming onscreen than they would have been on the page.

In addition to each cheerlady’s single-fiber plot thread, there’s one youthful subplot, about Cheryl’s grandson, Ben (Charlie Tahan, Love Is Strange), and the high school classmate he has a crush on, Chloe (Alisha Boe), who happens to be a cheerleader and is roped into helping the senior team prepare for the inevitable cheer competition.

And that’s what saves Poms from disaster — not the teenagers, but the contest finale, which is just as satisfying and fun as it is obvious. It’s as entertaining as the similar scene in Little Miss Sunshine, and only half as bawdy. If the idea of Diane Keaton leading a clique of graying cheerleaders makes you smile at all, Poms will be a pleasing post-Mother’s Day diversion for you and yours.

For most of its length, Poms is a C-minus movie, but that A-plus finale makes all the preceding lameness worth the toil.

Grade: B. Rated PG-13. Playing at the AMC River Hills, Biltmore Grande and Carolina Cinemark.

(Photo: STX Entertainment)