Annette Bening, left, and Julianne Moore star as longtime couple Nic and Jules in Lisa Cholodenko's domestic comedy The Kids Are All Right. (Focus Features/Alliance Films)

Three cheers for Lisa Cholodenko. After establishing her indie cred in High Art (1998), then stumbling in the uneven Laurel Canyon (2002), the writer-director makes a triumphant return with The Kids Are All Right, a smartly scripted comedy that depicts a modern-day family and ends up curing the summer-movie doldrums in the process.

One of the pleasures is seeing how the film gradually morphs from a breezy summer comedy into something far more dramatic and affecting.

Lesbian partners Nic (Annette Bening) and Jules (Julianne Moore) have been together so long, they've settled into a comfy yin and yang relationship. Nic is the tightly wound, successful ob/gyn and breadwinner, while Jules is a loosey-goosey, dithering underachiever who's just embarking on her umpteenth career, this time as an eco-friendly landscaper.

In the first of the movie's inspired family-dinner scenes, Nic and Jules are engaged in typical mealtime chatter with their two teenaged kids, 15-year-old son, Laser (Josh Hutcherson), and college-bound 18-year-old Joni (Mia Wasikowska). As the conversation drifts from a lecture about the importance of sending thank-you notes to concerned questions about Laser's deadbeat pal Clay, it becomes clear that Nic and Jules are every bit as square as Ozzie and Harriet.

The normalcy this bourgeois couple has worked to build is tested when Joni and Laser seek out Paul (Mark Ruffalo), the "donor Dad" whose sperm helped bring the kids into existence. Now the owner of a hip restaurant that serves up a very Californian organic menu, Paul is so laid back, he remains wonderfully unfazed by the unexpected news that he's a father: "Right on. I love lesbians!"

This David Bowie-loving free spirit quickly embraces his role as an alterna-dad, providing Laser with fatherly advice and inspiring model child Joni to challenge the strict rules enforced by her mothers. Jules begins to blossom in Paul's company, too, turning giddy in the presence of someone who appreciates both her beauty and gardening skills. Only Nic — whom Paul rightly identifies as "the gorilla of the family" — views the interloper as a threat to her offspring.

Paul's presence brings long-simmering tensions between Nic and Jules to the surface. The two women love each other, but the spark is gone — when Jules jokes "I can't breathe," during a hilarious under-the-covers sex scene, the offhand remark seems to be a comment on their partnership. Cholodenko and co-screenwriter Stuart Blumberg have a great ear for the subtle bickering that can arise in any long-term relationship. In a characteristically smart exchange, Jules cautions the tense Nic, "That's your fourth glass of wine."

"Actually, it's my third," her better half snips back, "but thanks for counting."

The chaos triggered by Paul's arrival is played mostly for comedy, some of it so broad that in a few scenes, The Kids Are All Right teeters on the brink of sitcom-y farce. Yet Cholodenko knows exactly when to quit horsing around, and ultimately has far too much affection for her characters to sell them out for mere punchlines.

It's difficult to single out a standout cast member. Wasikowska, for one, brings nice hints of quiet resentment to her role as the highly functioning daughter. The astute Ruffalo, meanwhile, turns in his finest performance in years. Paul could come off as a cad, a responsibility-phobic slacker cut from the same cloth as Ruffalo's character in You Can Count on Me. But the actor invests his hipster with unexpected yearnings and decency. Paul might suck at being a grown-up, but to his own amazement, he finds he'd really like to improve himself.

Laser (Josh Hutcherson, third from left) and Joni (Mia Wasikowska, second from right) meet their birth dad, Paul (Mark Ruffalo, right), in The Kids Are All Right. (Focus Features/Alliance Films)

As her previous films attest, Cholodenko saves her meatiest roles for her actresses. The finest moments in The Kids Are All Right come courtesy of Moore and Bening, whose names should crop up again come awards season. These two pros make great acting foils, using everything from their wardrobes to their speech patterns to suggest the differences that probably drew Jules and Nic together, traits that are starting to grate some 20 years on. To their credit, they create one of the more realistic, compelling portraits of a couple – gay or straight – in recent memory.

Bening gets the showier part, relishing every second of her hoot-worthy rants about heirloom tomatoes and Paul's unsolicited parenting advice. She's not afraid to make Nic unlikable, slowly peeling back the layers to reveal the control freak lurking beneath her too-tight smile. Nic emerges as the movie's most complex character, a fierce woman who will fight to protect all that she loves.

One of the deepest pleasures in The Kids Are All Right is seeing how it gradually morphs from a breezy summer comedy into something far more dramatic and affecting. This is a movie that creeps up on you — one minute, you'll be howling over one of Nic's zingy one-liners, and the next, Jules stops everything cold with a speech so moving, it had this reviewer reaching for the Kleenex.

Though Cholodenko takes pains to avoid any overt political commentary, she hasn't lost her edge. The movie has mainstream appeal, all while showcasing multi-faceted, relatable lesbian characters that are rarely given the big-screen treatment. Nic and Jules are far from radical, but the fact that Cholodenko gives these three-dimensional women their close-up nonetheless feels like cause for celebration.

The Kids Are All Right opens in Toronto on July 9, Vancouver and Montreal on July 16 and Calgary, Edmonton, Ottawa, Winnipeg, Victoria and Halifax on July 23.

Lee Ferguson writes about the arts for CBC News.