If any American musical underlines the idea that life is never what you planned, it’s “Falsettos,” which Penobscot Theatre Company is performing through Oct. 1 at the Bangor Opera House.

At the opening, Marvin has left his wife, Trina, and 12-year-old son, Jason, to take up with his gay lover, Whizzer. Because Marvin still wants a tight-knit family, and because the show takes places in the 1980s, there is also a psychiatrist who tries to help everyone grow into the new arrangement. Or at least sing a lot of songs in the process. In the meantime, the doc falls in love with Trina and creates a new family out of Marvin’s old one. Seems that everyone wants the thrill of love, even as the rules keep changing.

“Falsettos” is extracted from William Finn’s acclaimed trilogy of short chamber musicals based on Marvin and the civility he and his family circle cling to as the earth shifts under the weight of liberating times. As a composer and lyricist, Finn is most often compared to Stephen Sondheim, but his music sometimes gives a nod to the buoyancy of Cole Porter and has also been called a 1980s adult version of “Free to Be You and Me” – the 1970s family songbook and TV special about individuality, identity and acceptance.

“Free to Be” is not such a bad comparison. Like “Falsettos,” it is about the struggles of becoming a fully realized and happy human being in a sometimes nasty world, with relentlessly jingly songs about life’s trials and trips. The big difference is that the idealism of the earlier era spills into the eroticism and solipsism of the me generation in “Falsettos.” And just as everyone is settling into his or her place, AIDS enters as the story’s big aftershock, transforming silliness into sadness. The tragedy is surely one of the reasons the frame around Lex Liang’s window-like set is fractured.

Director Nathan Halvorson has gathered a bouncy and talented cast to deliver the comedy, rapture and pathos of the story. They can’t save the first act – “March of the Falsettos” – from the fact that Marvin starts out as one of the least appealing lead characters in musical history. But they adeptly set the stage for the second act – “Falsettoland” – to be a powerful display of friendship, acceptance and enlightenment.

It’s hard to say who leads the way in this strong ensemble. Andrew McCormick’s Teeny Tiny Band is anything but. Jeff Pierce (Marvin) and Martin Fox (Whizzer) harmonize in tenderness, strife and grief. Tina Marie Casamento (Trina) brings a big Broadway sound and all the nuances of a woman and mother whose longings get misplaced and tattered by the men around her. Although her outfits must be among the most unflattering and dowdy in designer Lex Liang’s costume shop (he has a particular affinity for plaid in this show), she completely embodies the role. Casamento is often very funny, and holds a stately balance when partnered with Scott R.C. Levy in a stand-up portrayal of Mendel the psychiatrist. A.J. Mooney and Katie Donohue burst onto stage in the second half as the chronically cheerful lesbians who live next door.

Young Ian Barrows turns in a surprisingly affecting and sincere performance as the dweebish son Jason. He doesn’t quite hit every note, but he nails every move of a child pushed and pulled by his parents’ crazy lives. He’s the one who moves the other characters – and the audience – toward understanding.

For those tempted to label “Falsettos” as simply a gay play, it’s worth noting that Finn stretches his themes in surprising ways – despite a cloying first hour. But if you make it through that opening half and if the miking of these mostly excellent voices doesn’t drive you nuts, stay for the second half. It’s the winning one. You may need tissues by the end, but you won’t be sorry you saw these characters achieve their best selves.

Penobscot Theatre Company presents “Falsettos” Thursday Sept. 28 through Sunday, Oct. 1 at the Bangor Opera House. For details, call 942-3333, or visit www.PenobscotTheatre.org.