Mia (Emma Stone) is a young, mostly out-of-work actress making a living as a barista on a studio backlot. Sebastian (Ryan Gosling) is a jazz pianist with exacting standards and a somewhat sour outlook on love.
According to formula, when they first encounter one another sparks fly, but they're argumentative and full of aggression.
As their relationship develops, it becomes a vessel for all manner of arguments about art, ambition, ideals and compromise, winding up in a place that, even if some viewers see it coming, will almost assuredly leave several of them in a cathartic puddle.
Working together for the third time, Stone and Gosling quickly establish an easy rapport, their surpassingly attractive physical features the perfect foils for Chazelle's aesthetic approach of naturalism and extreme stylisation. Neither is a particularly gifted singer or dancer, but that hardly matters in a film that sweeps them up as if carried by a swirling force of nature: They have the unforced grace of natural performers, lending an offhand rakishness to every step they take.
In addition to being fine actors in their own right, their gifts dovetail perfectly with composer Justin Hurwitz's ingenious songs, and have been lent even more sparkle by Tom Cross' crisp editing - which stays gratifyingly quiet during the gracefully filmed dance sequences.
One of the movie's themes is the often absurd pursuit of stardom that defines Los Angeles at its most shallow and careerist. Chazelle lards his script with little digs at showbiz jargon. (A young screenwriter Mia meets proudly announces his "knack for world-building".)
The film is literally inscribed with Hollywood's mythic past, from such familiar backdrops as the Griffith Observatory to the movie star murals on the city's streets. The subtext is that it has two stars at its centre who can convey hunger and avidity at one moment and a shiny sense of preordained fame and fortune the next.
But the real star in La La Land is the movie itself, which pulses and glows like a living thing in its own right, as if the MGM musicals of the Singin' in the Rain era had a love child with the more abstract confections of Jacques Demy, creating a new kind of self-aware genre that rewards the audience with the indulgences they crave - beautiful sets and costumes, fanciful staging and choreography, witty songs, escapist wish-fulfillment - while commenting on them from the sidelines.
In Chazelle's case, that commentary isn't ironic: It isn't delivered with pompous eye-rolls or scare quotes.
Instead, he harbours a genuine concern for a cinema that, in an age dominated by comic-book spectacles and stories dumbed down and miniaturised to fit an iPhone, is close to losing the scale and sweep and narrative values that defined and distinguished it in the first place.
Throughout La La Land, Gosling's character bemoans the state of jazz as a bastardised art form, further sullied by an audience indifferent to quality, originality and virtuosity.
It's difficult not to hear the film-maker himself in those words, anxiously observing how the art form he first fell in love with is undergoing existential transformation.
In La La Land, his answer his clear: The best way to deal with something that is shifting and changing under your feet - whether it's love, life or art - is to just keep dancing.
