Sarah Polley’s second feature as a writer-director, Take This Waltz, is a quiet and at times aching look at depression, dissatisfaction in marriage, and what happens when something new comes along. Michelle Williams, continuing her streak of portraying unhappy women, gives another impressive performance as the melancholy, 28-year-old Margot, alternately looking radiant and weary as she weathers the ever-so-subtle ups and downs of her five-year union and considers a fling.

Margot is mostly happy with Lou (a refreshingly mellow Seth Rogen), goofing around with him often with baby talk (which just skirts the line of intolerable), invented games, and lots of physical interaction. So initially she’s not interested when she meets the slightly antagonistic Daniel (Luke Kirby) while on a dull writing assignment and later just happens to sit next to him on a plane. The contrivances don’t stop there: Daniel is also her neighbor, living just across the street from her Toronto home.

Polley’s what-a-coincidence! scripting isn’t the brightly colored film’s biggest problem, however: If you’re going to make a movie about infidelity, you should probably make sure your leads exude a palpable attraction to each other. Kirby’s Daniel, besides having generic movie-star good looks, has zero charm or appeal, and it’s a head-scratcher why Margot is soon drawn to him. Her marriage is more realistically rendered, even if the pair are a bit too moody/pouty to be regarded as people mature enough to be hitched — though perhaps that’s the point.

However: Suspend disbelief! (Hey, you do it for action flicks.) There are a couple of scenes in which Williams and Kirby get the chemistry right, particularly when they go on an amusement park ride and are repeatedly tossed toward each other in the dark. (When the ride abruptly stops and the lights go up, Margot’s disappointment at being forced back to reality is all over her face.) And Margot’s ennui, both from her marriage and a lack of direction in general, will sink your heart, as will her tremendous guilt over her temptation to cheat.

The third act, too, is a devastating one, full of heartache, tears, and that guilt, which threatens to swallow Margot whole. Her husband is a good guy; he doesn’t deserve her desire to stray. The potentially life-sucking routine of long-term relationships is a strong theme here, with the climax proving that it’s practically inevitable, though one can choose to deal with it in different ways. Overall, Polley has created a delicately realized portrait of marital upheaval, even if you have to forgive a few missteps along the way.