Diane Keaton is “just your normal, overprotective, overbearing, over-the-top mother” in “Because I Said So,” which is just your basic humorless, irritating film trying to pass itself off as a charming and hilarious depiction of the volatile relationships between mothers and their daughters.

The makers of “Because I Said So” — the director Michael Lehmann and the screenwriters Karen Leigh Hopkins and Jessie Nelson — come from the “Diane Keaton-in-polkadots-makes-everything-cute” school of storytelling. There’s a reason you’ve never heard of it. Even Ms. Keaton and the adorable songstress Mandy Moore cannot make this film anything more than a saccharine, well-appointed disaster.

Ms. Keaton plays Daphne, a struggling single mother of three girls with an immaculate home and wardrobe. Though two of her daughters, Maggie (Lauren Graham) and Mae (Piper Perabo), have entered the perpetual bliss of matrimony, Daphne is convinced that her youngest daughter Milly (Ms. Moore), who has followed in her footsteps with a catering business, will end up an old spinster. The fact that the actress playing this role is a gorgeous 22-year-old does not seem to bother anyone involved.

Daphne, who is about to turn 60, is the only one ignorant of her attempts to use Milly’s “predicament” as a distraction from her own. A personal ad she has written in an attempt to set her daughter up draws the attention of two men. She conspires with a successful architect named Jason (Tom Everett Scott) to begin a relationship with Milly, but rebuffs a musician named Johnny (Gabriel Macht, playing the kind of marriage-minded bohemian type that exists only on celluloid) because of his unfortunate hand tattoo and her strange impression that he does not want to settle down. But Johnny has a 4-year-old child he raises on his own to prove his merit. The wily guitarist finds Milly on his own and pursues her anyway.

Ignorant of her mother’s plotting, Milly begins to date both men. Unable to choose between the two, she sleeps with both of them until the plot is ready for the whole thing to blow up in her face and get resolved. In the meantime, Daphne begins to date Johnny’s dad (Stephen Collins) and realizes that celibacy has made her kind of a jerk.

Throughout it all we are subjected to the kind of explicit sex talk that most daughters spend years trying to avoid having with their mothers. I blame Candace Bushnell. “Because I Said So” has inherited the basest aspects of “Sex and the City,” turning the sex-obsessed foursome of that series into a self-contained family unit.

Ms. Perabo is sacrificed as the oversexed, inappropriate family member early on, while Ms. Keaton and Ms. Moore work hard to salvage the scenes that attempt to exploit intimacy for humor. Only Ms. Graham escapes from her scenes unscathed. Slowing down the rapid-fire delivery she employs on “Gilmore Girls,” she makes Maggie witty, smart, and charming. But her slight role is only a small reprieve from the onslaught of neuroses meant to make us fall for the two leads.

Ms. Keaton has recently become something of a poster child for postmenopausal sexuality. But whereas 2003’s “Something’s Gotta Give” displayed the heart beneath her quirks, “Because I Said So” mistakes her spasticism for heart. Gratuitous shots of Ms. Keaton in her underwear may prove she still looks great, but they will not improve the script. Her billowing skirts and chic belts won’t make anyone want to watch her viewing Internet porn while her dog humps an ottoman. Similarly, no amount of polka dots will erase the searing image of Ms. Moore being forced to re-create an orgasm for her sex-starved screen mom. And not even the combined skills of Ms. Moore and Ms. Keaton can make the habit of speaking to your mother or daughter on the phone mid-coitus charming.

The restorative powers of cuteness only go so far, and this film never grasps the charming intimacy it’s striving for. With stilted dialogue and an excess of awkward moments, “Because I Said So” mistakes sex for tenderness, discomfort for intimacy, and gross-out humor for wit. There is a great film to be made about the dysfunctional and funny interactions between mothers and their daughters. But women contemplating seeing this movie should call their mother or daughter instead and take the moment to appreciate that their lives are not as painful as the ones depicted here.

mkeane@nysun.com