“Angels,” subtitled “A Gay Fantasia on National Themes,” is that rarity, an American play that finds ideas as sexy as, well, sex. Kushner sees America in a state of apocalyptic emergency, symbolized by John Conklin’s set, which features a classic-columned Federal facade riven by a huge jagged crack. That crack represents a fault line that extends from intimate human relations through the social fabric and on into heaven itself. Couples break up, society splits into opposing groups, God deserts his angels.

The couples are Prior Walter (Stephen Spinella) and Louis Ironson (Joe Mantello), two young New Yorkers, and Joe Pitt (Jeffrey King) and his wife, Harper (Cynthia Mace), two Mormons. When Prior contracts AIDS, the fearful Louis leaves him. Meanwhile, Joe, unable to deny his homosexuality any longer, leaves the emotionally troubled Harper. The leftist Louis becomes the lover of the Reaganite Joe; they are drawn to each other but excoriate each other’s politics. Joe’s mentor (brilliantly taken from reality) is the McCarthyite lawyer and ruthless power broker Roy Cohn (Ron Leibman). The homophobic Cohn is a closet gay who contracts AIDS (as actually happened) and is cared for by Belize (K. Todd Freeman), a black gay male nurse who despises the bigoted Cohn.

The “fantasia” of the subtitle refers to the play’s nonlinear structure, as it follows the shifting and interlocking relationships among the characters. Prior is desolated by the loss of Louis, who loves him but can’t conquer his fear of AIDS. Joe jitters with sexual ambivalence, repeatedly returning to and abandoning Harper. The pressure drives Harper into fantasies in which she travels to Antarctica to see the hole in the ozone layer that for her symbolizes the approaching breakdown of life. Her Mormon mother-in-law, Hannah (Kathleen Chalfant), comes from Utah to help her and is thrown into the moral chaos of New York. But Roy Cohn says: “God bless chaos.” He is the play’s satanic figure, and like Miltons Satan in “Paradise Lost,” he is the most compelling and appalling character, a vicious bigot who calls his nurse “a butterfingers spook faggot,” a power worshiper who voices respect for the “unkillable” crab lice that he catches from one of his pickups.

At the end of “Millennium an angel (Ellen McLaughlin) appears to Prior. This and subsequent angelic appearances in “Perestroika” are sensationally theatrical: “Very Steven Spielberg!” gasps Prior as the earth unzips and the angel zooms down like a jet-propelled Mary Poppins. She hails Prior as a new prophet, and the first part ends on this note of luminous ambiguity. In “Perestroika” the fantasia continues, moving toward resolutions that are tragic, tender and darkly comic. Cohn is irrepressible even in death, bursting into the afterlife to offer his legal services to the runaway God.

So who is Tony Kushner, and how did he come from nowhere to be the most talked-about playwright in two countries? HOW: With talent, inspiration and chutzpah. WHO: He’s 36, born in New York, raised in Cajun Louisiana, educated at Columbia University. He is gay, although he spent years in analysis trying to become straight before coming to accept his own nature. His only previously produced original work was “A Bright Room Called Day,” a play about 1930s Germany that blew nobody’s mind. Oskar Eustis of the Eureka Theatre in San Francisco saw Kushner’s potential and commissioned “Angels.” Now resident director at the Taper, Eustis (along with codirector Tony Taccone) has staged what over four years of writing has become a vast theatrical epic.

It’s a marvelously audience-friendly seven hours, but there are mush puddles and air pockets, especially in the newer “Perestroika.” And Eustis sometimes falters: the brilliant speech that opens Perestroika,” in which “the world’s oldest Bolshevik” mourns the death of Grand Theories, is played (by Kathleen Chalfant) too much for laughs with a thick Russian accent that obscures its elegiac power. Three performances stand out from a dedicated ensemble: Spinella’s gallant and poignant Prior, Freeman’s bitterly funny black nurse and Leibman’s Roy Cohn. Though he goes over the top at times, Leibman turns a pit-bull language of chronic rage into a perverse eloquence that is chilling. But the last word belongs to Prior, who rejects his mantle of heavenly prophet for his humanity-gay, doomed, gallant. “We won’t die any secret deaths anymore,” he says with quiet defiance. “And we will be citizens. The time has come. Get used to it.”