So much happens in the first act of Falsettos — not just plot, but scene changes, singing, music and character development — that at intermission, the woman in the seat next to me gasped in exhaustion. It feels like a more epic show than it is — like Les Miserables with neurotic Jews; call it Oy, Miserable!

William Finn’s two-act musical is actual two self-contained, one-act, 75-minute musicals played back-to-back with the same characters. Marvin (Max von Essen) divorces his wife Trina (Eden Espinosa) with whom he shares custody of their 10-year-old son Jason (Thatcher Jacobs on opening night), to move in with his hot younger lover Whizzer (Nick Adams). It’s 1979, when mixed families didn’t exist in the popular conscience … and neither did AIDS. Whizzer and Marvin enjoy a contentious but exciting relationship (best expressed in the Apache-style duet “Thrill of First Love”), but it cannot last; they break up by the end of Act 1. Meanwhile, Trina copes with the confusion, anger and challenges of this information (marvelously portrayed in the delightful patter song “I’m Breaking Down”) by shacking up with Marvin’s (and Jason’s) therapist Mendel (Nick Blaemire), who’s just as screwed up as his patients.

Act 2 takes place two years later, when Mendel and Trina are “a thing” and Marvin and Whizzer aren’t. Only Jason loves Whizzer, invites him to his ballgame and sparks rekindle feelings. But the tone shifts as a mysterious disease seems to be killing off gay men (“Something Bad is Happening,” a lively song with a foreboding message).

It’s significant to note that Falsettos opened on Broadway a year before Angels in America, Tony Kushner’s actual epic about life during the AIDS crisis, and get neither feel boxed in by their settings; the plots and characters seem as fresh and relevant today as they did decades ago. (Rent didn’t come for another three years.) That’s innovative enough, but the sung-through operetta impresses on many levels. Finn’s music tends to be sing-songy, but his complex lyrics and unusual rhyme schemes demand rival Sondheim’s; you have to lean in and stay alert to get everything there is to be got.

But what a joyous ride. The ending — one of the saddest in an musical — packs an emotional wallop, but Falsettos is as funny as it is profound, starting from the opening novelty number (“Four Jews in a Room Bitching”), until it rips your heart out {“Cancelling the Bar Mitzvah,” “What Would I Do?”). All the performances are spot-on, though it might be Jacobs as young Jason who steals the show from more seasoned adult actors. There is simply no end to the beauty of this show.

— Arnold Wayne Jones

Feb. 13 is Pride Night; performances continue through Sunday at the Winspear Opera House.