UNFINISHED and virtually unknown, Mozart’s opera “Zaide” arrived Wednesday night in a bizarrely modish production concept by Peter Sellars, providing a strange mixed blessing for the Mostly Mozart Festival.

A co-production with the Vienna Festival and London’s Barbican Theatre, it revealed all the virtues of great music – and all the follies of exhibitionist staging.

Mozart’s original is a story of passionate, thwarted love in a 16th-century Turkish harem between a recently captured slave, Gomatz, and the Sultan Soliman’s favorite, Zaide.

Sellars will have none of that.

Helped by designer George Tsypin’s silly cage of a setting, Sellars turns Mozart’s Turkish harem into today’s Western version of slavery – a sweatshop for illegal immigrants.

How arrogant to imagine that electric sewing machines, cellphones and a bucket of KFC add anything to Mozart. And how destructive, and futile, to try to impose some cheap kind of modern political pertinence on it.

Mozart composed “Zaide” in 1779, at age 23. Commissioned to write another opera, he put “Zaide” aside with its last act unfinished, leaving the distraught lovers (here sung by soprano Hyunah Yu and tenor Norman Shankle), aided by the Sultan’s overseer, Allazim (bass baritone Alfred Walker), pleading for mercy from the vengeful Soliman (tenor Russell Thomas).

It’s what is known as a singspiel (literally, “song-play”), the music broken up with spoken dialogue. Apart from a missing overture, Mozart’s unfinished score survived, but the linking narrative went missing.

Seventy-four years after Mozart’s death, “Zaide” was resuscitated in Frankfurt, with rewritten dialogue, an overture and a (happy) ending.

Sellars has had his own way with the dead Mozart – taking out the “spiel” from singspiel and leaving just the “sing.”

These arias (one or two known in concert versions), duets and a particularly wondrous closing quartet are ravishing – worthy of comparison with Mozart’s first operatic masterpiece, “Idomeneo.”

Sellars has linked these vocal passages with an overture and interludes taken from the composer’s contemporary music for a heroic drama “Thanos, King of Egypt.”

Again it’s marvelous, almost unknown, music. And again, Sellars’ staging manages to butcher it, using it for painfully long and awkward mime sequences cartoonishly synchronized to the score.

—

ZAIDE

The Rose Theatre at Jazz at Lincoln Center, West 61st Street and Broadway; (212) 721-6500. Performances tonight and tomorrow night.