Just three months before Lorraine Hansberry’s death in 1965, her eagerly awaited follow-up to Raisin in the Sun premiered on Broadway — and closed after only three months. (A 1972 revival lasted just five performances.) Now director Anne Kauffman has resurfaced Hansberry’s final work, an urgent, messy masterpiece called The Sign in Sidney Brustein’s Window that speaks even more boldly to our political and social culture nearly 60 years later.

Kauffman’s revelatory revival, which opened Monday at the Brooklyn Academy of Music, features a riveting central performance by Oscar Isaac as the title character, a Greenwich Village intellectual in the early ’60s whose lofty idealism often trip up his efforts to get ahead in terms of both personal and professional happiness. When we meet Sidney, he’s just failed at launching a folk-music club and is investing in a struggling alternative weekly that he intends to edit on the highest of commercial-free principles. He’s married to an aspiring actress, Iris (nicely played by Rachel Brosnahan), who has grown increasingly impatient with their stalled careers and yearns for both success and financial stability even as her affection for Sid lingers.

In these days long before the gentrification of the Village, the couple has surrounded themselves with similar left-leaning, low-earning strivers who would seem familiar to contemporary progressives: There’s the artist who’s designed the paper’s logo in type so small you have to squint to read it (Raphael Nash Thompson); the gay playwright who’s aiming to defy “Ibsenesque naturalism” by setting his new work entirely in a refrigerator (Glenn Fitzgerald); the perpetual underdog political candidate hoping to buck the Party machinery (Andy Grotelueschen); and the light-skinned Black intellectual who’s both an acolyte of Sidney and also willing to correct him on gaps in his education and worldview (“Your trouble, Sid, is that you admire the wrong parts of Thoreau”).

That last character, played by Julian De Niro (yes, son of Robert), is handed an utterly riveting Act 2 speech in which he dresses down Sidney in a way that punctures all of his tidy progressive pieties about race and romance. De Niro delivers the monologue with a kind of concentrated passion that is all the more devastating because of its restraint.

Julian De Niro and Miriam Silverman in “The Sign in Sidney Brustein’s Window” (Photo: Julieta Cervantes)

Yes, this is a play in which white liberals get their comeuppance — where their self-perception as fundamentally decent, broad-minded people is exposed as something just as short-sighted as the lives of the oppressors about whom they like to complain. (No wonder 1960s audiences didn’t warm to the play; I wonder if Bernie Sanders backers will be any warmer.)

One key counterpoint is provided by Iris’s older sister, Mavis (a spot-on Miriam Silverman), a representative of the often prejudiced, practical-minded bourgeoisie who swoops in from the suburbs to offer her own critique of the progressive movement. And in Hansberry’s generous hands, Mavis is no cartoon — and indeed she also needles Sidney for his hypocrisy about romance, preaching sexual freedom while demanding absolute fidelity of Iris. “Sometimes I think you kids down here believe your own notions of what the rest of the human race is like,” she tells him, after sharing how she has tolerated her husband’s mistress for years. “There are no squares, Sidney. Believe me when I tell you, everybody is his own hipster.”

The Sign is not without its flaws. Hansberry was already battling pancreatic cancer as the play was coming to Broadway, and had she been healthier the finished work might have been both tighter and sharper. There are a lot of storylines and themes in play here — arguably too many — and they culminate in a series of dramatic highlights in the second act rather than a single moment that brings the entire work into focus.

But Kauffman’s nearly perfect cast goes a long way to animate this long-lost gem — starting with Isaac’s electric performance as the hard-drinking, egotistical and self-deluding Sidney. In modern eyes, he might seem like even more of a monster than Hansberry intended — and yet Isaac makes him utterly compelling, a figure so mesmerizing that you can see why he has drawn Iris and Alton and the rest into his orbit. In Isaac’s hands, he becomes an avatar for Hansberry’s vision of the progressive intellectual scene: well-meaning, blinkered to reality and — most importantly — thoroughly and utterly human.