In Blue Valentine's most iconic sequence, a love-struck Ryan Gosling positions Michelle Williams underneath a heart-shaped wreath outside a florist shop and croons the Mills Brothers' "You Always Hurt the One You Love" while plucking a ukulele as she tap dances. It's their first date. You see the longing, curiosity and affection in their eyes. You want it to last forever.

You wish you hadn't seen the hour that preceded it. You know you'll soon return to the claustrophobic sex hotel where Gosling's Dean is trying to re-spark his marriage to Williams' Cindy. They've skipped town to stoke the dying embers of their once-burning flame. They're drunk and fighting. Their love is strained by his reluctance to grow up and her drowned ambitions: the two things that initially brought them together. You, and they, know their love is all but dead.

A film more than a decade in the making, Blue Valentine could well have been torture porn for the heart. Instead, director Derek Cianfrance has crafted a small miracle, a film that reminds us that movies are designed not just to stimulate, but to make us feel—even if those feelings are devastating. Seldom has a film had such an overwhelming sense of reality—aided by a searing soundtrack by Grizzly Bear, and by Cianfrance's choice to film early encounters with vibrancy and the hotel scenes with frigidity and claustrophobia. It's impossible not to care about Dean and Cindy, not to both sympathize and chastise their personalities.

Of course, none of the emotional impact of Blue Valentine would exist were it not for the shattering performances by Williams and Gosling, who each turn in the best work of their already impressive careers. Gosling's Dean is a charming layabout striving to be the best husband and father he can, but aspiring to little but chain smoking and drinking at work. Williams' Cindy is a blown spark plug of a woman who abandons her dreams of becoming a doctor to support Dean and their daughter. Their younger selves are drawn like magnets to one another, while their older selves are bloated and disheveled shadows who get along like fire and gunpowder. Each actor seems crushed under emotional weight, and that weight is shared with the audience.

Blue Valentine is not a film to be enjoyed, but neither is it overwhelmingly bleak. It shows us how sometimes the things we love become our own Achilles' heels, that sweetness can ultimately make the bitter sting even more. From the opening sequence to the beautifully rendered closing-credits montage, it clamps on the heart like an industrial vise, and keeps squeezing long after viewing. It's a triumph that stings to the core. R. AP KRYZA.

Blue Valentine

opens Friday at Fox Tower.