Charity Melville, late 20’s, overly dedicated and occasionally pedantic Ichthyologist. Seriously myopic, unconcerned about her coke-bottle glasses or ferociously wind-blown hair. Single, gay, attached to her fish, not any other human (at least for the moment). Leroy, her assistant, mid 30’s, newly hired, laid back, jovial, no real interest in science - it’s just a job. Content to follow orders. The two work on Catalina Island, California, where Charity tries to enhance the communication abilities of several fish.
Her star: Moby, an elephant nose fish, with a larger brain-to-body weight ratio than humans. Charity has inserted antennae in his brain and rear-end electrical receptor that significantly enhance his abilities to communicate with all fish, not just the dozen other experimental fish in her lab. They also magnify his natural ability to send out electrical charges. But no problem, as long as he stays in the lab.
Charity's aim is to help fish, safeguarding overfished species by augmenting their natural survival skills. Her boss, Ikan, a 60 year old Indonesian tycoon, has other plans. An amoral, anti-environmentalist, he plans to use her work to increase his company's yield of local fish, regardless of their dwindling numbers. But both their plans are seemingly foiled when Ikan's dim-witted son, thinking the experimental fish are dead, flushes them down the toilet.
The chase (and the mayhem) are on. Escaping the sewer system, Moby and his entourage whet their murderous teeth on a teenager violating (intentionally and unintentionally) the directive not to feed the fish. Then it's off to Avalon Harbor. A glass bottom boat ride becomes a nightmare. Then the passengers on an undersea submarine are treated to a terrifying underwater spectacle. Even the famed Catalina flying fish get in the act, air lifting a school of ravenous barracuda to attack the passengers on the Catalina Island Express Boat.
Always two steps behind, Charity posts reward posters seeking Moby's return. These (along with the person posting them) attract the attention of Jenny Hemingway, mid-20’s, gay, unattached (but looking to change that), dedicated California Fish and Game warden. Jenny has to track down this maniacal fish. Good luck with that. Moby continues to spread havoc. A night time attack on a spearfish charter by some loutish tourists. Then the owner of an exotic fish store meets his demise, along with his store and half the pier on which it was located.
Humans won't take this without a fight, though, sending Moby and his minions to the back side of the island where there's a fraternity/sorority fish fry in full swing. Moby enhances his legions with some moray eels, resulting in more blood and more deaths, but with some surprising survivors. Jenny and Charity arrive too late to stop it, so they head out to sea when they notice the spearfishing charter boat has returned. Only it is not there seeking to land a trophy fish; it is there for revenge. Alas (for them), Moby outwits them and their dynamite only results in their own deaths.
Charity, hoping to fix her creations, doesn't know how to get him back to the lab. Moby is always two steps ahead -- that is, until the U.S. Navy enters the picture. According to 40 year old naval intelligence commander Barry Wrigley, not only will they find and capture Moby, but they'll clean up the rusting barrels of toxic DDT that have plagued the waters (and fish) of Catalina for decades. Too good to be true? Well, duh, yeah.
They have other plans for Moby and his buddies, plans that include Charity being fired and Leroy arrested after he overhears Ikan and Barry talking. Moby, oblivious to all this, continues on his bloody path, enticing a small party of scuba divers into a cave where they become the main course. But Barry's ingenious plan is successful and soon Moby and his companions are back in the Charity-less lab. Viciously employing some shock training, Barry has Moby primed to begin his mission: lead fish into the toxic zone surrounding the sunken barrels, so that the now-poisoned fish can be recaptured and released off the coasts of enemy nations, devastating their economies.
Because Leroy is confined incommunicado, neither Charity nor Jenny knows of Barry's plan. They can't stop it. But Moby can. All it takes is some cross-species help from a cadre of dolphins. The plan is foiled, with Barry the only being to visit the toxic graveyard.
With the Navy gone, Ikan is no long protected from civil suits for the damage caused by his fish. He is forced to unload the lab to a non-profit, that hires back Charity and a recently-freed Leroy. But wait. Moby is still out there. And it is not only Charity, Jenny, and Leroy trying to find him, but Ikan, seeking revenge. And they all know just where to go: Bison Beach on the back side of the island, where Moby's troop has nibbled on some campers observing the buffalo.
Ikan arrives first. Armed with a speargun. It's a mano-a-fin showdown. Ikan doesn't have a chance. But as Moby slowly exits triumphantly, Charity leaps into the water. She caused all this; she has to fix it. Moby returns. Is he willing to forgive and forget his personal Dr. Frankenstein? Not quite. But as Charity sinks to the bottom, the cavalry in the persons of Jenny and Leroy save her and miraculously capture Moby.
Back at the lab and in Charity's love life, everything is going swimmingly -- until dim wit returns and, you guessed it, holds another fish funeral. We end with Moby eyeing his new playground: the Southern California coast.