This is an American, epic two-part play with a Jewish central character, probing the social and political legacy of the Aids epidemic that decimated the gay community in New York and beyond. And, no, it’s not a revival of Tony Kushner’s Angels in America.

But this new work, by Florida-born New Yorker Matthew Lopez, is almost as good and, at nearly seven hours combined, about as long.

Directed by Stephen Daldry (returning to the theatre after The Crown), the first three-and-a-half hours (with two intervals) is a dazzling display of coherent complexity. The second act’s attractions include a rare performance by Vanessa Redgrave who has the only female role in the play.

It opens with ten young men lolling on an almost prop-less plinth-like stage. They could be English literature students on a modern American campus on a summer’s day. Wannabe writers all, each has a story to tell though none has the inspiration to begin. It arrives in the incongruous form of a donnish English gent — E M Forster (Paul Hilton) no less. He is not only the inspiration to one of the students but also to Lopez whose play, we are told in the programme notes, is a response to one of the playwright’s favourite novels, Howard’s End.

With Forster’s help — a master-class in narrative decision-making — characters and plot are hammered out. Central is the Jewish, modest Eric Glass (Kyle Soller), whose boyfriend Toby Darling (Andrew Burnap) a fabulously arrogant memoirist whose blinding good looks and charisma are a danger to all who fall under his spell — not least Eric, whose grandfather was one of those who liberated Dachau and whose grandmother was a Holocaust survivor.

But it is through an older gay couple, billionaire Henry (John Benjamin Hickey) and Walter (Hilton again) that Lopez’s plot pulls in the legacy of Aids. It emerges that, in the ’80s, Walter helped those with the disease most of whom, we are reminded, were treated like lepers as they died in their thousands. Against Henry’s wishes, hundreds of gay men with the disease went one by one to die at Walter’s country house in upstate New York.

Lopez’s achievement can hardly be exaggerated. There are moments and characters in this play that have the eloquence and dramatic potency of Tennessee Williams. And, like Kushner, he seamlessly interweaves the personal and political, evident, for example, in the scene where Eric and his friends, hitherto certain of their progress in gay civil rights, experience the desolate reality check of Trump’s victory. And nor is Lopez afraid to collide the opinions held by America’s anti-Trump left and Henry, a Republican, who outs contemporary, left-wing anger as a kind of self-righteousness born of privilege, rather than the genuine grievances experienced by his generation.

But there are dramatic false steps here. All of them arrive in the form of what might be called an American talent for sobbing introspection: we are invited to place Toby’s narcissism, or Eric’s need to live a life of meaning — to name but two — on a par with the play’s wider themes. It’s a question of balancing those personal and political words, something which Kushner achieves with Angels, but Lopez doesn’t quite manage because too often he allows his heart to lead his writing.

So, the moment in which Toby saves himself from self-destruction by promising to be more honest with himself, feels awfully self-indulgent. And, almost fatally, this epic resolves in a kind of self-congratulatory cosiness that reduces much of what comes before. But then, most of what comes before is undoubtedly a triumph. And there is Redgrave, as the ageing mother of a son killed by Aids, giving one of most moving performances you will see this year.