Good rock opera isn't just born. It evolves. First comes a seed of inspiration, possibly triggered by mind-altering substances. Then it sprouts into a concept (something plausible, like a deaf, dumb and blind pinball champion). Next it is cultivated - the story line plotted, the characters developed and the songs written.

After that, it lays around on a shelf. When it does come up occasionally, much in the way last night's anchovies do, its creators make a face, wonder "what were we thinking?" and put it back on the shelf. Finally, the day comes when, with nothing better to do, with nothing to lose, with the realization that equally outlandish ideas have succeeded, the decision is made to show it to the world, or at least to Baltimore.

That may not be exactly how Tommy or Jesus Christ Superstar came to be, but it does explain the origins, pretty much, of The Giant Clam, a 19-song "rock opera" written by a local magazine editor and an animal pathologist.

Inspired by aquarium ornaments, latent childhood fears and, both writers admit, too much beer, the rock opera - dusted off, fine-tuned and rehearsed over the last several months - will premiere, five years after its creation, tomorrow at the Ottobar.

You could call it a rebirth - not just of The Giant Clam, but of the art form itself. But, depending on how you define it, the "rock opera" never totally died. A couple of cities even have rock opera companies, performing everything from Jesus Christ Superstar to rock-opera versions of Dracula and Macbeth.

Still, by the 1990s, more than 20 years after The Who started it all with Tommy in 1969, leading to a surge of rock musicals in the 1970s and beyond, true rock operas had become - like giant clams (tridacna gignas) - a threatened, if not endangered, species.

So it was only natural that Geoff Brown and John Keating, while sitting around one day pondering life's big mysteries - like why fake giant clams and miniature deep sea divers always showed up at the bottom of home aquariums, like why there was such a modern-day dearth of rock opera, like whether to order another beer - were struck, nearly simultaneously, by two thoughts:

What would happen if a diver got stuck in one of those giant clams? And why not write a rock opera about it?

As Brown, an editor at Baltimore magazine, explains it, "Stupidity took over from there."

Brown and Keating had been buddies since attending Johns Hopkins University together. After graduating in the early 1990s, they played together in a band called the Poorbillys, which performed a lot of original songs - including one written earlier by Keating called "The Giant Squid."

Keating, now 34 and living in Maine, said he has had a morbid fascination with denizens of the deep since childhood, the result, he suspects, of too much Jules Verne, too much television and those ornaments lurking in the bottom of home aquariums.

"It's hard to walk by an aquarium and see that clam with the pearl inside, and that diver, without getting the wheels turning," Keating said.

In reality, giant clams grow up to 4 feet in diameter, their pearls rarely are of value, and divers don't get trapped inside them. But for Keating and Brown, that didn't stop the creative process.

"You have to understand, it was around St. Paddy's Day, and there was a lot of Budweiser involved," Keating said.

Together, they came up with a rough story line: A poor fisherman, to feed his growing family, dives into the deep to pluck a giant pearl from a giant clam. Said clam clamps shut on him, trapping him inside. Efforts to rescue him ensue, involving a German diver with designs on the fisherman's wife, a giant eel, a giant squid, the Mollusk King and the pilot of a U.S. Coast Guard cutter.

They decided to each write 10 songs, further developing the story. To their surprise, when they got back together to compare notes, they were impressed with what they had come up with.

"They were good. We liked it ... but it never got off the ground," Brown said.

Years passed, and, except for talking about it and playing it among themselves, the "rock opera" languished. "Between us, it was like this legendary thing," Brown said, "even though nobody other than us ever heard of it."

"When they first came up with it, it just grew and grew and grew. And then it went away," recalled John Marsh, another member of the Poorbillys. "We would play the songs once in a while at practices to warm up."

Around 2000, the Poorbillys broke up. Keating, after earning his veterinary degree at Tufts University, moved to Maine to pursue a career in animal pathology. The remaining Poorbillys re-formed as the Billroys, all still keeping their day jobs. Brown, who plays guitar, is news and travel editor at Baltimore magazine. Marsh, the drummer, works for an Internet company. Bass player Bud Tiffany tends bar at Peter's Inn, the Fells Point Restaurant he and his wife own. Guitarist Neil Eber designs sprinkler systems.

Keating, though he had left Baltimore, stayed in touch, never giving up hope that The Giant Clam might some day surface.

Two years ago, he recorded 10 of the songs and sent them to Brown. "I think that at least kept the idea of doing something with it alive," he said.

Then, earlier this year, the Billroys, an alternative country band, started playing some of The Giant Clam songs again during practice - "just as a joke," Marsh said. "Then we all just looked at each other and said, 'Let's dust this thing off and see if we can do it.' "

"Somehow," Brown said, "as we got older, and it should have made less sense to do it, this strange and silly idea started making more sense."

First, deciding they needed a bigger sound for rock opera, they brought on a third guitarist, Chris Iseli, who freelances for Baltimore magazine and works in the communications department at Goucher College.

Then they proceeded to create a buzz.

They started a Web site (thegiantclam.com) and began placing mysterious ads - before a venue was even found - announcing that The Giant Clam was coming soon.

Brown assigned band members their roles: he as the giant eel and the German diver, Tiffany as the narrator and giant squid, Iseli as the Coast Guard cutter driver and Marsh as the Mollusk King and The Giant Clam.

"Geoff just handed out parts," Marsh, 32, said. He said, 'You're going to be the giant clam,' and I said OK."

Originally, he was going to wear an elaborate clam headdress during the performance, Marsh said, "but I couldn't wear it and play the drums."

The band began rehearsing in the spring. As of last week, with only days left before the performance, there were still a lot of details to be worked out.

They were trying to get fog and bubble machines for special effects, print programs with the words to all the songs, and settle on what sort of costumes to wear. Not until the end of last week did they finalize arrangements for a "showgirl" to walk across the stage between songs with signs announcing what was coming next.

Brown said there is an element of spoof in the production. It is, after all, rock opera.

"Rock opera ... is a pretty ludicrous concept," he said. "Don't get me wrong, The Who are just massively fantastic and managed to actually create ... rock operas that somehow made you nod and say, 'OK, sure, why not. ...'

"Keating and I were of the generation where [Pink Floyd's] The Wall and [The Who's] Tommy were something you listened to as a teen-age boy roughly 8,000 times," Brown added. "So we got the idea that rock opera was an acceptable, if silly, rock form in our heads at an impressionable age."

Since The Wall - though some might view it as more of a theme album - rock operas have been few and far between, despite some musicals and CDs billing themselves as such. The most recent is the Drive-By Truckers' Southern Rock Opera, on which all the songs are about the band Lynyrd Skynyrd.

But that work, some would argue, including more than one Billroy, fails to rise - or sink, depending on your point of view - to the level of true rock opera.

A rock opera purist, if there is such a thing, might argue that a rock opera must be a series of songs, with no dialogue in between, all contributing to the telling of one narrative.

Of course, that might be taking rock opera much too seriously, which the creators of The Giant Clam -though Brown says they did their best to follow the tenets of the art form - advise against.

"It's loud as hell, it's full of rock, and it's about a giant clam, for Pete's sake."

Performance

What: The Giant Clam (A Rock Opera) "World Global Premiere"

Where: Ottobar, 2549 N. Howard St.

When: Tomorrow. Doors open at 9 p.m.; Clam follows the opening act, "all-girl" band Vestal Vermin.

Tickets: $8 at the door or online at www.missiontix.com

Call: 410-752-6886.