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Say what you like about it, “The Impossible Dream” is one glorious diversionary tactic—a generous, stirring song whose soaring phrases help obscure the heart of the show it has come to represent.

Because here’s the truth: For all its low Sancho Panza comedy and its sublimely skanky chorus of flamenco-dancing bad guys, all its wanton gypsies and its scheming, singing womenfolk, Man of La Mancha is essentially a mournful thing about the nobility—and futility—of struggle. Sure, the show now camped out at the National Theatre is an expensively upholstered Broadway-bound revival, and a revival of a mildly creaky ’60s-era tuner at that, but as director Jonathan Kent has apparently noticed, it’s also the kind of musical that feels comfortable offering up the De Profundis as an eleven-o’clock number.

The song list calls it simply “The Psalm,” but the text the Padre chants over Don Quixote’s deathbed is unmistakably that of Psalm 129: “Out of the depths have I cried unto Thee, O Lord: Lord, hear my voice.” It’s an unusually desperate sentiment for a Broadway send-off, but what makes the moment especially striking is the image Kent creates to follow it up: As the strings swell and the lights warm for the final chorus, he sets the members of his ensemble climbing slowly, agonizingly up the rust-streaked walls of Paul Brown’s towering cistern of a set. They reach, and they strive, and they struggle—and they freeze, eternally aspiring, as the final chord echoes and the stage goes black.

But then Kent is something of a classicist—his credits for London’s Almeida Theatre range from Phedre to Coriolanus—so he’d know that there’s no triumph in Don Quixote’s story except the triumph of the spirit. Even the gently heroic soldier-poet Miguel de Cervantes (in this telling an Inquisition prisoner who spins the tale of Quixote’s curious quests in an effort to divert his bloodthirsty cellmates) seems headed for an unhappy fate at the final curtain: A pair of distinctly sinister jailers have come to escort him to his trial, and we all know how judicious those were under the Inquisition.

Taking its cues from those darker threads in Dale Wasserman’s adaptation of the story, Kent’s production avails itself of every opportunity to remind its audience of the human squalor that so disturbs Cervantes’ mad cavalier in the first place: Its grime-streaked chorus, its raunchy choreography, that bleak industrial set are all of a piece with Quixote’s assessment of the evil wizard he believes is at large in the world: “He has eyes like little machines, and where he walks the earth is blighted.” That’s a phrase of harsh poetry indeed, and it rings considerably harsher in memory when Quixote finally locates his nemesis—glaring back at him from within a mirror. The best thing about Kent’s production, whatever its failings, is that it does justice to the bleakness at the story’s core.

It does have failings, though. It feels long, for one thing, at roughly two intermissionless hours, and it seems overfond of technical gimmickry—set pieces are constantly rotating, descending, or catching fire. (A friend observes, though, that Brown’s magnificent spiral stair is the best set of onstage steps since Hello, Dolly!, and that’s probably a fair assessment.) Richard Amaro’s choreography, a presumably unintentional counterbalance for all the flash, is strictly run-of-the-mill.

And yes, Brian Stokes Mitchell possesses arguably the theater’s most glorious baritone, which he unfurls to considerable effect in songs such as “Dulcinea” (delivered with so many brushes of mezza-voce that it’s practically one extended sigh) and “The Impossible Dream,” to which musical director Robert Billig has added a handy key change designed to showcase Mitchell’s money notes. But he can be a bit of a ham as a musical-theater actor (as opposed to his “straight” work in shows such as August Wilson’s King Hedley II)—and this part requires his Cervantes to spend the bulk of the evening pretending to be an endearingly mad old man. The result is occasionally as cutesy as you might expect.

As the object of his chaste devotion—Aldonza, a tavern wench initially unimpressed by Quixote’s insistence on seeing her as his chivalric ideal—Mary Elizabeth Mastrantonio presents the opposite problem. Her acting is relatively subtle, and a few moments shimmer with real magic, but her singing voice is unfortunately uneven—so much so that casting her opposite Mitchell seems like cruelty. It’s two voices, actually, with a thin, poorly supported upper register joined to an undistinguished middle by a chasm of a break that she clearly has difficulty singing across—hardly what you want for Aldonza’s more plaintive material.

Of course, Ernie Sabella can’t sing at all, but this show’s take on Sancho Panza is all comic relief, so it hardly matters, and Sabella’s clowning is first-rate. (Ask any child who’s giggled at his flatulent Pumbaa in the animated Lion King.) Among the supporting cast, Stephen Bogardus invests the usually disagreeable twinned characters of the Duke and Carrasco with considerable charisma and even a degree of humanity; Mark Jacoby, another fine actor, is both funny and warm as the priest. And Natascia Diaz, the vibrant Petra in the Kennedy Center’s summer production of A Little Night Music, shows similar spark as Quixote’s daughter, whose mortification at the old man’s antics helps bring about his downfall.

What’s best about Kent’s take on the Knight of the Woeful Countenance, though, is his sensitivity to what little subtext remains in Wasserman’s reduction of the sprawling original. (La Mancha covers only the first six chapters or so of Cervantes’ novel.) One of Mitchell’s first big speeches is about the power of imagination, of performance, of theater. And when he begins his first song—when he commits his own first act of theater in that dark Inquisition prison—Kent and Brown respond with a staging coup that underlines the point with surprising grace and substantial power. It’s drama distilled to its very essence—and for such flashes of inspiration, many theatrical sins may be forgiven. CP