With “operatic” now a synonym for excessive grandiosity, and the demanding diva de rigueur for any opera, it is easy to forget the primal, visceral nature of the early years of the art. To paraphrase what is said in the fascinating new Broadway play “Farinelli and the King,” which is playing at the Belasco Theatre in New York City, opera once was a much dirtier business. Not so much galas, donors and elitism as candles, sweat and story.

Those latter three nouns are one way to sum up what this strikingly original new play from Shakespeare’s Globe in London — penned by Claire van Kampen, performed mostly in ambient candlelight and starring van Kampen’s husband, Mark Rylance — has to offer. But it’s also a piece that asks whether music can restore your sanity.

What music, you wonder? Well, the show, which features live Handel arias and the accompaniment of a baroque ensemble, holds a candle for the tri-octave art of the castrati, that most fascinating and terrifyingly self-sacrifical of vocal genres and, this play posits, the art with the most potent power to calm a fevered brow or assuage a wandering soul.

Rylance (“Bridge of Spies” and “Dunkirk”) plays Philippe V of Spain, the grandson of the intimidating Louis XIV and, by his own description, an aberration of a king. When we first meet Philippe, he is fishing from a goldfish bowl. Like you do. Or like you do if you are Rylance, who dives deep inside wacky Philippe in a performance as empathetic as it is seemingly spontaneous.

Farinelli was an acclaimed Italian castrato of the 18th century, obliged at a young age to be castrated in order to pursue his art and increase the size and purity of his vocal range, and to improve the economic prospects of his family. “Farinelli and the King” explores whether the former can save the latter — as engineered by Philippe’s savvy and complex Queen Isabella (played by Melody Grove, who clearly gets the many sides of her lady and yet shrewdly never fully reveals the Queen’s hand).

Castrati now being thin on the ground on either side of the Atlantic, the great Farinelli is sung in this London import by the great British countertenor Iestyn Davies, although the character is simultaneously acted by Sam Crane (the two performers stand side by side, Davies showing up whenever Farinelli sings). This is a strange device in practice, and you wonder why Davies, actually no slouch as an actor, did not just play both roles, demanding as the work of a countertenor may be.

The royal couple become increasingly at one with both parts of Farinelli, whose particular vocal situation does not impede pleasures derived in other areas of his life. Whether he can save the king’s sanity is, for sure, the main question of the play, which is directed by the rising John Dove, but “Farinelli and the King” also is very much about opera’s own rise amid questions of European identity and inter-continental aggression.

“Farinelli and the King,” a strange and slow-burning theatrical experience in many ways and seemingly focused on just one relationship, actually turns out to be a remarkably complicated exploration of the most important question in the arts of the last 500 years, i.e., who gets to go?