The rarely performed "Candide," now at 5th Avenue, is a rare treat for lovers of opera, operetta and Broadway musical comedy and is a celebration of how far Seattle musical theater has come.

How do you solve a problem like “Candide”? Based on the classic Voltaire saga of innocence lost, the musical failed on Broadway in 1956. But many have aimed to rescue its enchanting, exhilarating Leonard Bernstein score by revising and restaging this unusual comic-operatic epic into a less unwieldy work.

Arguably the best revised version is one crafted by John Caird for London’s National Theatre in 1999.

5th Avenue Theatre has turned that sly, cogent rendition (closer to Voltaire than to the Lillian Hellman text) into a triumphant centerpiece of the Seattle Celebrates Bernstein festival.

A rare treat for lovers of opera, operetta and Broadway musical comedy, this is also one of the finest undertakings of artistic honcho David Armstrong’s 5th Avenue tenure.

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Three hours long, with a single set, chamber orchestrations, an episodic romp through historical calamity and philosophical absurdity, it is a demanding adventure. (And with its lusty, bleak and gruesome goings-on, not one for the kiddies.)

But Armstrong’s agile staging flies — from the delightful overture dispatched with gusto by Joel Fram and orchestra, to the exhilarating choral capper, “Make Our Garden Grow.”

Set designer Matthew Smucker’s assemblage of outsize geometric components, which change in ingenious ways in relation to Tom Sturge’s deep-color lighting effects, sets a blithe panhistorical tone. So do Lynda L. Salsbury’s ’50s-esque costumes.

But it’s the principal actors and sterling chorus that make this “Candide” soar.

Bernstein’s amazing score, with its nods to Mozart, Rossini, Offenbach and Strauss, and its literate lyrics (largely by Richard Wilbur), set the vocal bar sky-high. Clasically trained voices are essential here.

Chorally and individually, that bar is met by a company led by the terrific tenor Stanley Bahorek (a perfectly boyish Candide), and glamorous soprano Laura Griffith, a wonder as Candide’s great love, Cunegonde.

Griffith’s sublime rendition of the most famous tune, the killer bel canto aria “Glitter and Be Gay,” truly brings down the house.

As the lovers wend their separate ways through a wicked world of wars, inquisitions and greed, fleeting pleasures and ironic double-crosses, their progress is narrated by a wry, amiable David Pichette (doubling as optimism-peddling tutor Dr. Pangloss and Voltaire).

Anne Allgood, fine singer and spry comedian, is a hoot as a sassy old servant (“I Am Easily Assimilated”), while Mike McGowan is an able-voiced hunk of Teutonic beefcake as a snotty noble.

With local top-liners like Allen Fitzpatrick, Brandon O’Neill, Billie Wildrick and others in the smaller roles and the chorus, this “Candide” is also a celebration of how far Seattle’s musical-theater scene has come — and what, at full strength, it is capable of.

Misha Berson: