Early in their courtship, Dean (Ryan Gosling) serenades Cindy (Michelle Williams) on a city street, late at night, with the pop standard “You Only Hurt the One You Love”. By the time we see this moment, in flashback, it’s already quite clear that, in the present, they’re making good on that song’s title.

But something else interesting is going on beneath the surface of the youthful romantic exuberance of the scene. Dean tells Cindy that in order to sing, he’s got to do so in a funny voice — and right there, he transforms himself into a kind of corny lounge singer. Whether intentional or not, it underlines one of the major themes of Blue Valentine: while love can be transformative, what it transforms us into can come as a big surprise.

Director Derek Cianfrance’s film is an emotionally bracing, startlingly honest look at what a difference just a few years can make in two lives that have been joined together. For people who have, themselves, felt a relationship dissolve just this hopelessly, it may be direct and visceral enough to elicit the cold sweats of post-traumatic stress flashbacks. And flashbacks are exactly what Cianfrance uses to unfold this story, using them more effectively than any film in recent memory.