I didn’t know quite what to expect from Billy Elliot: The Musical.

I hadn’t seen the much-beloved 2000 film on which it’s based, and all I knew of the stage adaptation was the frenetic excerpt performed at the 2009 Tonys, during which Billy hauled in 10 awards, including Best Musical (though not one for Elton John’s ebullient score).

As for the subject matter — a 12-year-old boy in a North East England coal-mining family finds his inner ballet dancer against a backdrop of labor unrest and Thatcherism in the mid-’80s — it seemed an unlikely mix of gritty and twee.

Well, unlikely it is. And that’s one of its many resounding virtues. Yes, the milieu (and the language) are gritty, but the details, right down to the dowdy cardigans on the miners’ wives, are exquisitely conceived. And when worlds collide — as in the number “Solidarity,” in which the paths of ballet students, miners and cops intersect, or in “Expressing Yourself,” when the fantasies of Billy and his cross-dressing pal Michael explode into a zany chorus line of gigantic headless frocks — well, the whole thing soars unexpectedly into the realm of the fantastic.

Yet the performances, under Stephen Daldry’s seamless direction, always feel grounded and specific, right down to the littlest members of the ensemble.