In “Mothers and Sons,” Terrence McNally’s well-acted, sometimes touching new play, Tyne Daly as Katharine Gerard pays an unexpected visit to Cal (Frederick Weller) the lover of her son Andre, who died long ago from AIDS.  To understand why McNally wrote “Mothers and Sons,” it is necessary to go back 25 years. To understand why it doesn’t quite work as a play on Broadway, one must linger at the curtain call.

In 1988, McNally wrote a brief monologue that he expanded two years later into “Andre’s Mother,” an episode of American Playhouse on TV starring Sada Thompson and Richard Thomas that showed a memorial service where Katharine Gerard, the dead man’s grieving mother, would not speak to Cal, Andre’s grieving lover.

McNally has decided to write something of a sequel for the stage.  Cal is now a money manager living in a beautiful apartment on Central Park West with his husband Will (Bobby Steggert) and their seven-year-old son Bud (Grayson Taylor.) Katharine is now a recent widow, and she has traveled from her home in Dallas on her way to a European vacation, and dropped by unannounced, having not seen nor communicated with Cal for decades, although she has kept in touch with Cal’s sister.

Over the 95 minutes of “Mothers and Sons,” McNally uses the four characters to explore a range of issues and updates.

There is much talk of the novelty and marvel of two men now being able to be wed. (In real life, it has been three years since New York State legally allowed same-sex marriage.)

“Andre and I were what people called boyfriends then,” Cal says. “Or partners. Lovers was another word people used. We didn’t like any of them. Boyfriends sounded like teenagers, partners sounded like a law firm and lovers sounded illicit. They all seemed insubstantial, inadequate. Then along came the new- but-old-and-obvious name for it. It’d been there all along: husband.”

There is talk of gay fatherhood. ‘The sight of two men hand in hand with a child waiting for the light to change in Central Park West rattles an occasional cage,” Will tells Katharine. “Gay dads still merit more than passing interest even in the metropolis known as Manhattan.”

There is an exploration of the guilt, grief, rage and resentment that comes with loss.

“It will never be ‘our’ loss,” Katharine snaps at Cal. “You lost a man and quickly found other men. I lost a son.”

There is also a consideration of homophobia — “After all these years, it still sickens me,” Katherine says about gay love – and whether there is any possibility of change and reconciliation.

All four performers are first-rate, and do what they can to make their words more than positions. But my trouble with this play seemed to come into focus at the curtain call for the performance I saw. Bobby Steggert made a plea to the audience to support Broadway Cares/ Equity Fights AIDS. This was not because the play was centered on a character who died of AIDS. Such solicitations from Broadway stages to fight AIDS are routine, and have been for more than two decades. Yet “Mothers and Sons,” despite its interludes of eloquence and feeling, too often seems written for a different audience – an audience in great need of enlightenment about AIDS and issues affecting gay people. The script is not so much preachy as obvious.

The character of Katharine offers some intriguing possibilities. Credit McNally for making her something more than just a bigoted yokel; she has a subscription to the New Yorker magazine, and in the infrequent times she is in New York, she stays at the Algonquin. She says “I don’t understand how my life turned out like this,” but what little we know about her life makes clear she has always been unhappy.

Would that McNally had explored Katharine or his other characters in “Mothers and Sons” with the same depth as the characters in some of the other plays in his 50-year career as a playwright – “Frankie and Johnny in the Clair de Lune,” “Master Class,” and his trio of pioneering plays about gay life, “The Lisbon Traviata”, “Lips Together, Teeth Apart”  and “Love! Valour! Compassion!”