This is the story of a well-known playwright and an unknown technical director; of Israel Horovitz, who thinks enough of fish to write a play about quite a large one, and Kerry Comerford, who considers them "slimy creatures all, I can't stand to touch them" but had to construct the whopper. It is a story bursting with emotion, passion and missed deadlines, possibly the greatest fish story ever told simply because it involves the biggest fish ever put on an American stage. Period.

That would be the quarter-million-pound mackerel that is the obvious center of attention in Horovitz's new play, the aptly titled "Mackerel," starting previews Wednesday night at the Folger. When Louis Scheeder, producer of the Folger Theater Group and "Mackerel's director, first read the script, "I said, 'We could never do this, this is crazy' and then I called Israel the next day and said yes."

That conundrum characteristizes the awful appeal of "Mackerel": It seemed impossible to build an acceptable fish in an acceptable period of time of an acceptable cost, but how to resist the challenge, how to back off from the potential fun? No one could, least of all Kerry Comerford, whose several all-nighters with the beast characterized the Andy Hardyesque spirit that went into its construction.

"I've never been so excited about a project in my life," he said as he began. "Technicians have dreams and nightmares about things like this. All scenery is big toys, but I never imagined I'd do something like this."

"This" in final form weighs close to a ton, and as the fish sits backstage at the Folger waiting for its big moment, its plexiglass eyes seeing all and seeing nothing, it seems more ominous than playful, an eerie, forbidding visitor from Conan the Barbarian's Hyperborian Age. Charlie the Tuna it is not.

Four members of the Lemon family - father Ed, mother Emma, daughters Edna and Eileen - share the Horovitz play and the Folger's cramped stage space with the behemoth. They all live in a ramshackle house in Gloucester, Mass., where Ed moved the clan from Kansas because, Emma says scornfully, he had "a dream that God had a fortune for him in the ocean." Unfortunately, since the move "he hasn't seen a fish outside of a can in months now," and the family apparently hasn't engaged in much in the interim except to get on each other's nerves something fierce.

Suddenly, just at the beginning of Act. One, Scene Three, a savage hurricane sends That Fish crashing through the wall of the Lemon dwelling. "The great fish undulates ever so slightly," the script reads. "It's breathing, slow deep-throated gasps are heard, barely." The Lemons are appropriately thunderstruck. "This," says Ed, "is God's biggest mackerel." What will a mackerel in the living room do to the Lemon's lives, not to mention their cleaning bill? Therein lies the drama. "I Hate 'Em"

Louis Scheeder has been in charge of the Folger Theater Group since he was 26 years old. An intense, staccato man, he gave the theater what Newsweek called "a reputation for youthful audacity" by bringing in plays like "Medal of Honor Rag" and "Black Elk Speaks." Louis Scheeder is now 31. "Mature, dignified," he says almost to himself. "No more enfant terrible. "

What Scheeder likes in plays in general, and in "Mackerel" in particular, is the quality of "saying something about various societal conditions. This is a play about asking God for a gift and being given the most marvelous thing, something no one was ever given before. And then what do you do with it? There's a richness of texture here. It's not a message dealing with one thing. Israel's writing about the world."

Sure Louis, but what about The Fish? "I know it sounds insane," he says, "but believe me we've had some very heavy arguments about that fish. The fish does not exist in isolation, it's a character, it responds back and forth. We have to decide what the personality of this fish is; is it bemused, terrified, scared? The shop could make the most wonderful fish in the world and it might not work for the play."

Sitting across from scheeder is the personification of the shop, Kerry Comerford, a wiry 27-year-old with curly red hair who is in his second season as the head of the Folger's sevenman construction crew. Previous to this the most exciting thing he'd built was a huge Fiberglas tree for "Midsummer Night's Dream." When he first heard inklings of the big fish, "I just laughed. I didn't think Louis was serious. I said, 'Yeah, okay, what's the next script?'"

Making things so much the worse was Comerford's intense personal dislike of all things fishy. "I hate 'em, I hate to touch 'em," he says, shuddering. Yet in the interest of theatrical realism he'd bought two bluefish - "mackerel was out of season but they said this was a close cousin" - and carefully scrutinized them: "I boiled 'em down to see the skeleton and they fell apart. I found out a lot about the structure of the fish, but it was a mess."

As a result of this detailed study Comerford concluded that if the mackerel was built as big as it should be "it would go from stage right through the library all the way to ther other end of the block." That obviously would never do, so he decided to "cheat a little bit" and make the final dimensions of as much of the fish as would be seen - from the front of the mouth to its movable gills - 10 feet long, 9 1/2 feet high and five feet wide."It's big," the builder says cheerfully. "It'll upstage everybody." Deep-Throated Gasps

The fish was born in a gray garage-like building on Archibald Walk SE, home of the Folger's shop. It began, with its black metal superstructure, looking something like an SST. The addition of wood made it resemble the world's biggest mouse, and it was not until the foam rubber was put on that a fish-like shape was discernable.

Two weeks into March, Kerry Comerford took some time out from ramrodding the fish to explain how he hoped it would work. The white foam that went on top of the wood would fill out the flesh and give the fish its general shape. Gray foam on top of that would give a smooth surface, on which would be placed devcon, a liquid rubber compound usually in demand for fixing gaskets but here used to make the fish simultaneously slick, slimy and waterproof. Flame retardant salts would be put on top of the devcon, and then the whole thing would be painted appropriately fishy colors.

The plan was to have the fish come apart into about 20 different pieces so it could be gotten through the Folger's modest-sized loading doors. It was to make its appearance by being pushed through a breakaway wall, with an actor sitting inside to operate the various head-turning levers and make those slow, deep-throated gasps. Among the problems yet to be solved was how to deal with the fish in the second act, when the script calls for it to begin to decay. Louis Scheeder had recently discovered Slime, a gloppy kid's toy that he thought might be ideal, but no final decision had been reached.

"I'm not giving any promises on this one," Comerford said, surveying the fish-in-progress. "If one joint freezes up because of misalignemnt, it could freeze up the whole thing. So many things can go wrong on this show, it astounds me." A Coating of Slime

On March 17, the Mattel toy people, makers of Slime announced that they would donate 165 gallons of the wretched stuff to help make the mackerel look rotten. "I don't know about the goodness of our hearts," said Joel Rubinstein, Mattel's director of marketing and public relations. "I think we're a little looney.

"This whole thing is absurd, you know this," Rubinstein went on, adding that previous requests for Slime have been for more mundane projects such as a sound effect in recording studios, as soil for growing avocado seeds, even one from an orthopedist who felt convinced that frozen Slime "made an excellent pack for wounded muscles."

Unfortunately for the Folger, it would have to make do with last year's model Slime. New Slime has more of a purple than a green look, Rubinstein explained, and, appropriately lowering his voice, he added, "It has another ingredient: worms." The Human Factor

Preparations for the human participants in "Mackerel" took place in a modest rehearsal hall in the linoleumtiled basement of the Capitol Hill Presbyterian Church. A makeshift kitchen was et up, tape marks placed on the floor, and the actors, at work on the opening scenes, are doing a lot of screaming, groaning and all-out emoting. "We're just digging to find the reailty on which to base the comic stuff," explains Louis Scheeder, looking a bit like a mad scientist launching a runaway experiment. Building the fish is apparently lots more fun that acting with it.

By the third week in March, pressures on the fish folks were increasing and people were starting to worry about Comerford. "Kerry tends to get really involved and wound up in things," said Peet Foster, the master carpenter. "If somebody doesn't say, 'Hey, when was the last time you ate?' he ends up getting sick."

Among the problems were a delay at the foam rubber factory which meant that Comverford had to dash around Washington buying $530 worth of foam at various outlets. Then the contact cement used to apply it refused to work, and the devcon liquid rubber shipment turned out to be late as well. Topping it all off, a decision was made not to take the fish apart into 20 pieces but to instead move it through Capitol Hill on its very own Sears casters. Once arrived at the theater, it would be hoisted up on scaffolding built from the group's "Hamlet" set and taken in through a little-used side patio entrance.

"This is a tight one, it's hysterical, I'm a little slap-happy," Comerford admitted cheerfully as the deadlines approached. "The fish is starting to get there and I can't get it off my mind. I'm just very anxious, let's put it that way." Mr. Mackerel

The curriculum vitae handed out by Israel Horovitz's agent is a full five double-spaced pages long in which the reader learns that Horovitz has written a huge lot of plays which have enjoyed, in addition to other honors, "highly successful productions in Japanese, Serbo-Croatian, Swedish, Greek and Hebrew."

Ten years ago, Horovitz told Newsweek he'd written "a million plays, an embarrassing number." Confronted with that number now, he half smiles and says, "I lie, I can't be trusted." No exact figure comes to mind, he says. "People can count 'em up when I'm dead, Possibly I've written one too many, but I don't know which one."

Sharp, puckish, deadpan, with eyes that tend toward the woeful, a very nattily dressed Horovitz has come down from Hartford to lunch in Washington on March 24. "It took me an hour to get the luggage," he says as he settles down with his salad. "That's what life comes down to Luggage."

He wrote "Mackerel," he says, as one of a series of comic items undertaken as relief from a more somber cycle known as the Wakefield plays. "I tend to be extremely grim when I'm writing those," he adds, "I go into deep depressions, long silences, it's unpleasant to be with me, especially while I'm alone."

"Mackerel," which he characterizes as "German symbolist theater of the 17th century crossed with 'Close Encounters of the Third Kind,'" is, he feels, "very dark while it's being funny. It never stops being a serious play." He turns to his teen-age son Matthew and asks, "What do you think?" and when Matthew responds that "it never stops being serious but it's silly," Horovitz promptly agrees and says, "It's silly."

Horovitz was up in Hartford overseeing a workshop-type production of "Mackerel." Their fish, he confides, "doesn't look like a fish. It's kind of a Hartford fish." Of the Washington model he says humbly, "This fish is more than I deserve." Still, appreciation goes only so far, for Horovitz plans to miss the fish's outdoor excursion. "The fish moving on the street," he concludes, "is of little consequence in my life."

What is important, however, is running. "I'm here ostensibly to work on a play," he says, but the real reason is a chance to run in the annual Cherry Blossom race. "I run to balance the time at the typewriter. If I get up, I eat. I wrote a play about a woman named Edith Tempt who ate 17,500 calories of raisins a day and had to run 60 miles to stay thin. We later learn that she also ate the Boston Red Sox and a statue of Voltaire."

Lunch is over and the Horovitzes rise to leave. The playwright's final words are, "I hope you won't be tough on Matthew." Parade's End

Escorted by two bemused motorcycle policemen, pushed by a dozen or so people, pulled by a Scout driven by Adam Foster, the master carpenter's father and ironically a member of the Potomac River Commission, the fish made the trip to the Folger on March 29 with all deliberate speed and no discernible mishaps. It went so fast, in fact, that like all great theatrical moments, it was over almost before it'd begun.

One of the pushers was an apprentice actor named Jimmy Dean who would be the man inside the fish, leading to jokes on the order of "James Dean Is 'Macherel.'" 'I'm 25 years old," he says, "and it's my first fish." What does he do for motivation? "I get a lot of bait thrown in." Breakthrough!

At 10:30 P.m. Saturday April 1, only four days before the scheduled start of previews and almost a week later than everyone would have liked, a hushed crowd of technicians and crew watched as the mackerel was for the very first pushed through the breakaway wall.

"It kinda burst through like an explosion, it freaked me right out," said properties master Charles Woolverton. "All of a sudden it glides out of a semi-blackout and all my plates flew off the shelves. It's spectacular."

Kerry Comerfold, a bit dazed even a day later, was grateful for the success. "We really did it, and the first time," he said. "It looked, oh, fantastic. There was applause. It made me feel good."

The fish was ready bu the people were less cooperative. On April 4, the day before public previews were scheduled to start, the Folger announced a parting that would mean a one-week delay in previews until a new actor could be found and rehearsed and an indefinite delay in the official opening date.

For Kerry Comerford, there was a certain irony in the cause of that delay. "It's not technical," he said over the phone, and one could almost hear an unseen smile.

Still, Comerford was grateful for the breathing space. "There's cosmetic stuff I'd like to rework, parts of the upper lip, the eye, underneath the fish. It looks pretty much like a fish and everything, and it works, but on this kind of project, you really set your heart on its being perfect. I'd just like it to be that way."