The playwright Charles Busch made a name for himself writing (and playing the drag heroines of ) genre send-ups like “Vampire Lesbians of Sodom.” Of late, however, Mr. Busch has been going mainstream — first with his hit Broadway comedy “The Tale of the Allergist’s Wife” (2000), and now with something less auspicious: a surprisingly tame period piece about theater folk called “Our Leading Lady,” which opened Monday night at City Center.

Mr. Busch’s latest follows the popular actress Laura Keene (Kate Mulgrew) and her co-stars in “My American Cousin” as they prepare to perform at Ford’s Theatre on the night that — unbeknownst to these troupers — President Lincoln will be assassinated by John Wilkes Booth. Despite its historical backdrop, however, “Our Leading Lady” is all backstage squabbles and power plays; its pressing concerns include whether the actors will be able to get their trunks out of their police-padlocked dressing rooms.

Manhattan Theater Club has lavished resources on “Our Leading Lady” — Lynne Meadows directs, Jane Greenwood designed the crinolines and lace bodices, and Santo Loquasto created the elegant, functional Civil War-era set. But despite the trappings, Mr. Busch’s play can’t sustain interest over its two sluggish hours.

The main trouble is that “Our Leading Lady” can’t commit to an identity. Is it a broad backstage comedy? It certainly begins that way, with a bombardment of one-liners that hit more often than they miss. This is pleasant enough stock material — the scheming diva (Ms. Mulgrew), her sour-faced rival Verbena (Kristine Nielsen), and Verbena’s wayward husband (Reed Birney), who lingers afterhours with the prentice boys. Throw in a ditzy matriarch (Barbara Bryne), a suitor for our leading lady (Maxwell Caulfield), a jaded ingenue (Amy Rutberg), and an escaped slave disguised as a Chinese servant (Ann Duquesnay) — and you’ve got the makings of a knockoff Oscar Wilde farce.

But the frothy first half ends with the shooting of the president, sending the second half into a tailspin. What emerges is a 1950s Douglas Sirk melodrama, with Miss Keene’s black maid speaking earnestly to her boss about relations between the classes, and Miss Keene finding it in her hardened heart to be forthright, honest, and compassionate with her long-abused supporting players.

The two halves don’t fit together tonally, and there isn’t much Ms. Meadows can do about it. It’s a bit of a shock when “Our Leading Lady” makes a sharp turn from backstage farce into race-relations drama, and it’s equally disconcerting when a police inspector shows up to question the actors and walks into a sort of Agatha Christie scene laced with yuk-yuk jokes.

The only continuity between these shifting genres is provided by our leading lady, Ms. Mulgrew, whose Laura Keene — a curious blend of her Katharine Hepburn (from “Tea at Five”) and an rrolling Judi Dench — exudes a strapping life force. Miss Keene, a Cockney barmaid who abandoned two daughters and went onstage to escape a brute of a husband, proves to be a solid citizen underneath all her layers of vanity. She’s a diva you can grow to love.

This is bad luck, alas, for the play — Miss Keene was a lot more fun as a fire-breathing dragon. Early on, she’s described as a “great big shining sugar-glazed ham,” but she’s a ham in a good way — an outsize comic presence in a play sorely in need of one. Later, when Miss Keene gets in touch with her humanity, she becomes rather sentimental, and the play grows dull. It’s as if, having set up the ground rules of his own comedy, Mr. Busch decides in mid-stream not to play his own game.

There’s a sense of acute discomfort in watching Mr. Busch, who has played so many of his own leading ladies to camp perfection, trying to write a sincere heroine. He ends up applying little wit or sparkle to the protagonist, and still less to the genres — there is nothing here approaching either a sendup or a valentine. In “Our Leading Lady,” Mr. Busch seems to have shut down his more fecund faculties and stuck to the straight and narrow. One can only ask, with consternation: To what purpose?

Open run (West 55th Street, between Sixth and Seventh avenues, 212-581-1212).