In Trousers, currently in production at the Lyric Stage, is a small play with big ambitions. In its Boston premiere, the William Finn musical is the first of the "Marvin" trilogy that Finn and James Lapine went on to complete, and while Falsettos, the Tony Award-winning combination of the latter two plays, has received much acclaim, the first one-act has merits all its own.

The show opens and closes with the same musical number, "In Trousers, the Dream," but in the intervening one and a half hours, twenty-six songs take us through "Marvin's Regression," as we watch Marvin turn fourteen, break hearts, fall in love with women, fall in love with men, get married for ten years, have a son, have seizures, have breakfast, have sex and fantasize recurringly about being Christopher Columbus.

It's a show that isn't dependent on plot surprises, or even plot development for that matter--from the outset, Marvin declares his crisis over his personal and sexual identity, and the rest is just a matter of assessing how the people in his life have figured into it. Tim Ewing astutely plays Marvin as a man with his life just barely under his control at all times, a seizure, literal or metaphorical, possible at any moment, alternately full of energy and anxiety, lusty happiness and distraught sleeplessness.

His wife Trina is played by Julia Kiley, who hits some of the more pathos-laden notes in songs such as "I Felt him Slipping Away" and "Breakfast over Sugar" as she tries to keep Marvin from leaving her. As Marvin's high school teacher, Rosemary Loar is generally funny in her over-the-top characterizations of the schoolmarmish yet sexually hungry Mrs. Goldberg, but her voice occasionally seems too thin. Julie Dixon seems less adept than the other two women in switching between being Marvin's high school sweetheart and being one of the chorus: while Loar and Kiley are distinctive as Marvin's wife and teacher and occasionally even let those roles seep into their chorus girl functions, it seems as though Dixon lets her chorus girl demeanor overtake her role as Marvin's high school sweetheart. Her voice is strong but her character colorless.

All four members of the cast have strong stage experience behind them and deliver rousing renditions of Finn's score. Accompanied solely by percussionist Paula Rich and pianist Gerald Moshell, the cast's collective performance stand up to the demands of musical theater for a dynamic multiplicity of talents. Moshell, too, is the embodiment of a multiplicity of talents--playing piano is the least of his contributions to "In Trousers." Moshell holds full production and direction responsibilities for the play's Boston run, as he did when he staged the first-ever production of the entire trilogy of "Marvin musicals" last November in Hartford.

The staging itself has a lot in common with the Broadway staging of Falsettos. Both employ many bright-colored modular building-block pieces on rollers that become tables and chairs and footstools and playthings. Choreographer Paul J. Tines smoothly incorporated props into his lively numbers, especially in songs like "How Marvin Eats His Breakfast" and "Whizzer Going Down."

In Trousers lacks a certain substantiality and maturity that Falsettos contains, but it also thankfully lacks the "Cosby-Show" digestibility that made Falsettos such a hit. Falsettos seemed preoccupied with a "family values" message, even if that meant "alternative family values" that included son and father and gay lover and lesbian caterers.

In Trousers worries less about the moral of the story, and so while the story seems zany and rough around the edges and occasionally incoherent, it has more texture, more nerve, more craziness. In not trying so pointedly to convince you that these characters are normal or at least lovable, you come up with that realization all by yourself.