Vincent Lacoste in Lolo (David Koskas/FilmRise)

Love triangles are confusing enough, but when one point of the diagram happens to be a scheming, possessive son, affairs of the heart move in a truly twisted direction. Slapstick meets The Bad Seed in Julie Delpy’s Lolo, a fitfully charming tale, marked by contradictory impulses and abrupt, bewildering shifts in tone.

We first meet high-strung fashionista Violette (Julie Delpy, restless and mercurial) on a spa holiday with her confidante Ariane (Karin Viard), exchanging the kind of bad-tempered, foul-mouthed grousing only best friends can get away with. Their repartee recalls a more down-to-earth Patsy and Edina from Absolutely Fabulous or the gals from Sex and the City, with a rougher attitude and about 25 more IQ points. It’s the best thing in the movie and never fails to rivet when other scenes sag.

Horny and bored, the two Parisians look for some action with a pair of local guys. After Jean-René (Dany Boon) accidentally drops a freshly caught tuna in her lap, Violette finds herself unexpectedly falling for him. And pourquoi pas? Jean-René may be no sophisticate, and the film makes much of his gauche cluelessness, but he is employed, warm, loveable, and impressively endowed. Best of all, he’s moving to Paris, so their affair can continue.

Unfortunately, there’s a long, skinny obstacle who likes to lounge around in his underpants standing in their way: Violette’s pretentious, cooler-than-thou teenaged son, Eloi (Vincent Lacoste), who’s stuck with the infantilizing nickname Lolo. The sneering Lolo is detestable from the get-go, and he will instigate increasingly more outrageous actions to drive Jean-René far, far away from maman.

Lolo loses its way as the tension mounts in this fraught ménage. The elaborate torments Lolo inflicts on Jean-René are hit-and-miss funny at first, then improbable, then over-the-top cruel—only a psycho could take such a strong dislike to decent, generous Jean-René. Since we don’t really get inside Lolo’s mind, we don’t know what his motives are other than a routine jealousy of his mother’s suitor (or suitors, as it turns out). The savvy, on-point Violette somehow can’t see her son’s constant pushing of her limits. When a long-simmering confrontation between her lover and her precious boy finally erupts in violence, Violette rolls her eyes skyward and faints. Like us, maybe she can’t figure out to laugh out loud or scream in terror.

For all its indecision about what kind of movie it wants to be, Lolo dispenses some worldly delights. Delpy and Boon show off warm odd-couple chemistry, set to a retro soundtrack. Offhand dialogue sparkles with wit, and for all its fawning over Paris’s creative-class scene (with a cameo by Karl Lagerfeld), the movie suggests that down-to-earth values matter most. By the time a light turn-and-turnabout plot twist comes into view at the end, you may be willing to cut Lolo some slack and wish Delpy a surer touch in her next effort.