The voices of a cappella quartet Anonymous 4 presented a straightforward simplicity in Wednesday’s concert. (Chris Carroll/Kennedy Center)

Four women sit on a sculpted wooden platform dressed in simple, vaguely medieval gowns of wool and silvery gray. They sing gently, voices moving together, intoned words overlapping and echoing one another, the music gently nudged into different shapes by subtle electronic enhancement, tinged with the tocks and thwacks of glockenspiel and drum. It’s exquisite, hypnotic, watery, dreamlike.

The piece is David Lang’s “Love Fail,” which the Fortas Chamber Music Concerts co-commissioned and brought to the Kennedy Center’s Terrace Theater, performed by the wildly popular a cappella group Anonymous 4, on Wednesday night. “Love Fail” is epic on a couple of levels. For one thing, it lasts for a long time: a solid hour of gentle music, monumental in Lang’s terms. For another, it evokes other long and large works of the past: from medieval tales declaimed by minstrels to the looming form of its main musical antecedent, Richard Wagner’s opera “Tristan und Isolde.”

“Love Fail” takes Tristan and Isolde as its subject — but the story, not the opera. Most of the work’s 12 sections are distilled from different retellings of the original legend, which has been stripped of its characters and narrative to leave us with individual vignettes into the viscera of love.

“If I drown, I know that you will drown,” sings one of the women, continuing the thought into a litany of the shared fate two mystically linked lovers might meet. Contrasting with this intensity are short stories by Lydia Davis talking about the mechanics of love — one section discusses how, in a debate between lovers, being too right can be wrong.

All of this is set to music that feels like a distillation: light and intense, with a faint taste and a strong afterkick, like aquavit. Lang, once associated with brasher and louder pieces, has come to specialize in polished, gemlike vocal works, particularly since the success of “The Little Match Girl Passion,” which won the composer the 2008 Pulitzer Prize and a Grammy award and catapulted him to a whole new level of recognition. Shortly before Wednesday’s concert, he was announced as the holder of the Debs Composer’s Chair at Carnegie Hall for the 2013-14 season.

Having mixed it up with the modern world for most of his career — the composer’s collective he co-founded, Bang on a Can, is more commonly seen at the intersection of contemporary music and indie rock than as associated with early music — Lang is now examining the way that world can interact with some of the cornerstones of the classical canon: Bach Passions, Wagner’s “Tristan” or Schubert Lieder, the basis of Lang’s recent Carnegie Hall commission “Death Speaks.”

Wagner’s theater, as well as his subject, is distilled in “Love Fail,” which is a work of theater in which the theatrical elements — gently shifting lighting by Jennifer Tipton; subtle soundscapes by Jody Elff; sculptural costumes by Suzanne Bocanegra; video snippets by Jim Findlay that often look like still photographs, until the men and women in them blink — are so subtle that they create an existential question about what constitutes a work of theater.

This is theater stripped of its drama, love stripped of its passion, singing stripped of its opera: the four distinctive voices of Anonymous 4, each a little different, present a straightforward simplicity that’s a far cry from the heft of a Wagnerian soprano. Even Wagner’s words are thinned away; the final movement of “Love Fail” is based on the text of the famous “Liebestod” that concludes Wagner’s “Tristan,” and the words “Mild und leise” float, defused, as “mild and light.”

The result is heady — even cerebral. “Love Fail” presents not operatic passion, but the way that real love works in the lives of real people: a quiet radiance rather than a burning flame. It may make you laugh, but it will probably not make you cry. Lang has talked about abandoning his earlier ironic stance and daring to present himself honestly, and certainly there was something naked, even naive, about “The Little Match Girl Passion.” That sense has carried over into “Love Fail,” but here the nakedness is contained within such a solid, well-crafted covering that you may be beguiled by its exterior surfaces rather than penetrating to its core.