Six years after Sarah Polley's powerful directorial debut "Away From Her," she's back with another warts-and-all look at committed relationships. But unlike the couple who had been together for decades in Polley's first film, "Take This Waltz" focuses on a young wife and husband who are still at a relatively early, and fragile, stage in their marriage.

From its youthful focus to its whimsical dialogue and twentysomething female protagonist, "Take This Waltz" initially feels like exactly the sort of film you'd expect to see from a fledgling actress-turned-director. In other words, a film worlds away from the bold and mature surprises Polley delivered in "Away From Her." Fortunately, this is no sophomore slump. "Waltz" grows deeper as it goes, and it soon becomes evident that Polley isn't just fulfilling an ambitious actor's wish to call the shots. She's got the real goods for the job.

Polley also has the great fortune to have three-time Oscar nominee Michelle Williams as her leading lady. Williams delivers another fearless and exceptional performance as 28-year-old Canadian Margot who finds herself irresistibly attracted to bad boy neighbor Daniel (Luke Kirby) despite the five year martial bond she shares with nice guy husband Lou (Seth Rogen). Instead of throwing herself into an affair, Margot wrestles with the concept. She meets up with Daniel on an increasingly frequent basis to indulge her fantasies without taking action, and whether or not she'll cross the line remains an open question for most of the film.

Once again, Polley proves herself a natural filmmaker -- occasionally laying on the visual metaphors a little thick but also discovering a wealth of beautiful images to illustrate her small-scale story. Margot is a frustrating character, but she's not an unsympathetic one. Even as some of the dialogue spells out her dilemmas a little too clearly -- "I'm afraid of being afraid," she tells Daniel when they first meet; "You seem restless," he tells her later; "New things get old," she observes in a gym shower, surrounded by casually naked older women -- both Polley and Williams are so intent on exploring this woman with compassion and understanding that the film never devolves into a morality or cautionary tale.

There are two sequences late in the film that make use of music to illuminate a character's psychological state as expertly as any film in recent memory -- one set to Leonard Cohen's "Take This Waltz" and another to The Buggles' "Video Killed the Radio Star." They're Polley's bravura moments, crafted with invaluable assists from returning "Away From Her" director of photography Luc Montpellier and new editor Christopher Donaldson. And they'd be almost inconceivable without Williams carrying the movie. It's already well established that Williams is one of the best actresses of her generation, and the way she throws herself into this role without concern for Margot's likability yet never losing grasp of her inner struggle is further proof of that fact. Whether or not she receives a fourth Oscar nomination for this film, the performance is among her most challenging and complex and she absolutely aces it.

The men in Margot's life are a study in contrasts. Rogen delivers one of his most charming performances to date as Margot's sympathetic hubby, while Kirby is faintly unappealing as cocky player Daniel. Rogen is the less conventionally good looking but more charismatic actor, which puts an unusual spin on Margot's conflict. There's still a clear divide between the safe "boring" choice and the dangerous "exciting" option, but the casting doesn't overwhelm the characterizations. Even though Kirby is the weakest performer in the core cast, he nails a sexually explicit monologue at the center of one of his trickiest scenes that's key to making Margot's attraction credible. Sarah Silverman rounds out the key players in a terrific supporting turn as Lou's alcoholic sister who shares a special bond with Margot.

When "Take This Waltz" premiered at the Toronto Film Festival last year, it provoked divisive reactions from audiences and critics. It's not an easy film to embrace because its characters are not easy to embrace. But there's no question that Polley cares about these people -- she doesn't judge them, she wants to understand them. And that impulse is increasingly rare in movies today.

"Take This Waltz" is now available on demand and opens in limited release June 29