It must have seemed like a good idea — or at least a lucrative one — at the time.

Blend Anglophobe populism right out of “Braveheart” with a bit of “Wicked”-style revisionist girl power. Toss in memories of last year’s most popular film (pirates!) and of last year’s most memorable film performance (Queen Elizabeth! Well, the first one!). Then get the tunesmiths behind “Les Misérables” and “Miss Saigon” to write it and the duo behind the Irish-dance megalith “Riverdance” to produce it. Can’t miss, right?

“The Pirate Queen,” as it happens, misses by a nautical mile. An indifferently constructed blend of period pageantry, power ballads, and battle-of-the-sexes pandering, this soggy melodrama does little to stir the pulse, to say nothing of rehabilitating the rusty image of the pop opera. Alain Boublil and Claude-Michel Schönberg helped pioneer that genre in the 1980s; they, along with many collaborators, have turned their attentions to a little-known historical tidbit: One of Ireland’s clans — a disparate group united by little beyond a common enemy, the usurping forces of England — was led by a woman named Grania “Grace” O’Malley in the 16th century.

While Grace (Stephanie J. Block) roams Ireland’s western coast wreaking havoc on British ships, a newly crowned Elizabeth I (Linda Balgord) fumes over this unexpected dent in the Empire. And so the battle lines are drawn: Virgin Queen versus Pirate Queen, full royal regalia versus sweaty tank top, upper-limits legitimate soprano versus upper-limits belting.

Both rulers also must contend with institutional sexism: covert in Elizabeth’s case, all too overt in Grace’s. Buoyed only by the stalwart support of her lover, Tiernan (Hadley Fraser), Grace triumphs over the prejudices of her peers at least a half-dozen times. This culminates with a priceless Act 2 battle in which Grace staggers onto the deck minutes after giving birth to grab a sword and start stabbing.

That description may promise a camp extravaganza, the sort of cross-dressing tour de farce that Charles Busch might write as a vehicle for himself. But “The Pirate Queen” plays it far too safe, falling back on a deadening love-triangle melodrama. (The busy direction is credited to Frank Galati, although Graciela Daniele is reported to have taken over.)

Messrs. Boublil and Schönberg never have placed a high premium on stylistic verisimilitude. The “Pirate Queen” score sounds like their typical, unapologetically melodic, fare seasoned with a dusting of Irish instrumentation straight out of James Horner’s “Titanic” score. Still, it’s no less genuine than the duo’s previous evocations of 19th-century France and 1970s Vietnam, and nobody minded much then. More worrisome is their willingness to crib blatantly from “Les Misérables.”

With the exception of a sturdy ballad for Grace at the end of Act 1 and a final confrontation between the queens, the alternately syrupy and boisterous score rarely rises to even the lesser moments of “Les Misérables.” The lyrics by Mr. Boublil, his “Miss Saigon” collaborator Richard Maltby Jr. and John Dempsey range from the serviceable to the squirm-inducing.

Things perk up whenever the action shifts to England. Messrs. Boublil and Schonberg have traditionally relied on the lower classes for their comic relief, but here the play’s few laughs sit on the Virgin Queen’s elaborately outfitted shoulders: “There is something, father, / That you never said: / Did you ever admire someone / That you would like dead?” Luckily, Ms. Balgord launches into the role with all the semi-ironical authority she can muster.

Not every actor is so lucky. Jeff McCarthy peacocks gamely through the role of Grace’s stern but loving dad. Marcus Chait is passably hissworthy as the cowardly Donal, who wins Grace’s hand for tactical purposes, and Mr. Fraser brings a comfortable virility and stirring tenor to the role of Donal’s romantic rival.

Ms. Block, who contributed a memorable Liza Minnelli in “The Boy From Oz” without sliding into mere imitation, draws a tougher assignment here. Perhaps in homage to the score’s “Titanic” connection, she approaches her many ballads with a hooded vibrato and pronounced swoop that is more than a little reminiscent of Celine Dion. Ms. Block labors to climb out from behind Grace’s ringing chest tones and constant feats of derring-do but is hogtied by her two-dimensional role.

Ms. Daniele and her creative team are so intent on plowing through that they race past a handful of compelling visuals. A stage-spanning sail left in tatters after a defeat, the positioning of actors high atop set designer Eugene Lee’s shipboard riggings: These and many other images warrant more time than they receive. Only Queen Elizabeth’s increasingly elaborate dresses, courtesy of costume designer Martin Pakledinaz, receive any sort of lingering attention.

With three choreographers involved with the production, “Pirate Queen” is the most dance-heavy pop opera to reach Broadway since “Cats.” But unlike that much-maligned musical, these dance scenes act as little more than window dressing, albeit stirringly executed window dressing.

Three communal events — a wedding, a funeral, and a christening — serve as a somewhat flimsy pretext for Irish-dance sequences, buoyed by a large “Riverdance” contingent among the chorus members. Although these provide much of the show’s high points, common sense frequently takes a powder during them. The scenes in which male dancers use wooden oars to stamp out rhythms on the ground are impressive, but might not one of those three choreographers have pointed out that pirate ships don’t have oars? This forsaking of logic is not exclusive to the visuals. Late in the show, one character makes this offer on behalf of a jailed lover:

Prepared to tender
My unconditional surrender …
Let me trade my life for hers
And I’ll surrender all.

There’s a name for a surrender made under such terms. It’s called a conditional surrender. When a musical with a title such as “The Pirate Queen” inspires thoughts of oars and prefixes, rather than romance and danger, surrender would appear to be the appropriate response.

Open run (213 W. 42nd St., between Broadway and Eighth Avenue, 212-307-4100).