The late Stephen Sondheim’s upcoming final musical, “Here We Are,” is the biggest mystery of the theater season.

Audiences know almost nothing about this should-be major cultural event by one of the greatest composers of all time that’s only weeks away.

But one of its many puzzles is a particular head-scratcher. 

“Here We Are,” according to sources familiar with the book and score, has hardly any songs in its second act.

One insisted it has “zero,” adding, “Steve bailed because he could not write Act 2. He gave it up and never thought it would be done.”

The insider went on to say that “there are only six songs in Act 1, and one is repeated in every single scene.”

Talent agents also sent actors to be seen for nonsinging roles in the musical, which ended up with a starry cast including David Hyde Pierce and Bobby Cannavale.

When asked if Act 2 has songs, production spokesman Rick Miramontez would not reveal any specifics about the show.

“Here We Are,” which is directed by Joe Mantello and starts previews Sept. 28 at the Shed in Hudson Yards, is based on two films by Spanish surrealist director Luis Buñuel — 1972’s “The Discreet Charm of the Bourgeoisie” and ’62’s “The Exterminating Angel.” 

Act 1 is said to be “Discreet Charm,” while Act 2 is “Angel.” The second part is heavily reliant on playwright David Ives’ book.

The legendary creator of “Sweeney Todd,” “Into the Woods,” “Company” and “Sunday in the Park With George” died in late 2021 at 91, and his final musical, in whatever state it’s in, will be required viewing for Sondheim buffs.

But many have doubts that it’s ready for prime time.

“This is half a show,” a source said.

Yet one that’s charging full price. Theatergoers have griped about the stratospheric ticket prices, as the quality of the musical is unknown and it’s based on material unfamiliar to many.

Seats are $129 if you’re lucky, and $350 if you’re not. That’s more than Saturday night at “Hamilton” and “The Lion King” on Broadway.

Those eye-gouging costs for the 15-week run are due, according to a source, to its humongous $8 million enhancement. 

“Enhancement money” allows commercial producers to subsidize their own shows at nonprofit theaters. It’s a practice that’s become commonplace, but that $8 million amount, the source said, is one of the largest ever.

“Here We Are” is produced by Tom Kirdahy, Sue Wagner, John Johnson and the Stephen Sondheim Trust.

As for the music that is in the show, how is it?

One source who’d heard the score said it “has several terrific melodies.” Another said that “it’s not great Sondheim” and sounds “like David Yazbek if translated from the French,” referring to the composer of “The Band’s Visit.”

Everybody agrees “Here We Are” is unwieldy and half-finished.

But one industry vet said Sondheim, who reportedly shelved the incomplete musical seven months before he died, became reinvested later and was about to embark on the next phase of the long-gestating project. They suggested he might have even given instructions for the piece’s future to his collaborators. 

New Sondheim music being out in the world is a good thing — though ideally for less than $350 a pop.

And this will be the first new Sondheim musical since “Road Show” opened to negative reviews at the Public Theater in 2008.

We’ll see what the critics have to say about “Here We Are” when it opens on Oct. 22.