Rovert Ludlum, the author of over half a dozen best-selling supense novels, likes to tell this tale about how he stumbled onto the plot to "The Rhineman Exchange," his fourth book that was recently stuffed and mounted as an "NBC Best Seller."

He was on the beach near his home in the Virgin Islands, a place he visits so rarely that he is now trying to sell it, when a man approached him. The year was 1970, only a few days before Ludlum's first book, "The Scarlatti Inheritance," was to be published.

The man, well over 6 feet, leaned down and said quietly, "Are you Robert Ludlum? Did you write "The Scarlatti Inheritance?"

"You read it?" Ludlum asked, thinking he had gotten a review copy somehow.

"Yes," the man said, he had been sent a copy by a government agency to "vet" for security problems. "You have a very conspiratorial mind. Supposing I were to give you four points of history to consider."

Ludlum listened. "Fact No. 1," the man said, "in January 1944, at Pennemunde, the Germans ran out of industrial diamonds. Fact No. 2; the B-17 bomber came off the assembly lines here with a less-than-adequate gyrocompass. Fact No. 3; the Germans had already perfected a gyro-compass, Fact No. 4; the Allies had access to industrial diamonds."

"Are you suggesting this actually happended?," Ludlum said.

"I just want you to think about it," the man replied.

"That's obscene," Ludlum said, "two super powers at war, trading in military hardware. It would make a great book."

"No," the man said, "no book must ever be written about it."

Then, he stood up and left Ludlum alone on the beach.

Ludlum refuses to give the man's name even today, seven years later.

But he has two theories. "Either something like this was on the drawing board someplace," he says, "or this man had a wild imagination."

Now Ludlum is pretty sure it was a little of both. "When I finally got around to doing the research for 'Rhineman,'" he says, "I ran across something that terrified me. In 1944, the Germans went into French Equatorial Africa to guard a source of industrial diamonds. Sperry-Rand had perfected a gyro-compass under a top-secret project in 1946. Everybody knows the war ended in 1945, so what were we using before that?"

He lets the question hang suspended for a moment, as if to emphasize the point that his suspense novels have a curious way of mixing fact and fiction that sometimes surprises even him.

It is a particular vision that has paid off handsomely.

Since 1970, Robert Ludlum's books have sold over 10 million copies in 22 countries. "The Chancellor Manuscript," his most recent book, has just gone into its fourth printing and has sold 105,000 copies in hardcover. He recently signed a three-book deal with Bantam for seven figures. A television producer interested in making the change to feature films wants to buy the book he is working on right now, one he hasn't even finished yet.

His paranoia is obviously contagious. When "Scarlatti" was published - with the interesting premise that an internationald group of money men joined together to back the growing Nazi party for profit - he says the Pentagon felt obliged to deny it.

"Only the Pentagon would deny something that doesn't exist," Ludlum says cheerfully.

His books, however, are not always total fantasy. Take "The Osterman Weekend" his second book which dealt with the domestic operations of the CIA. It was written in 1971, years before Seymour Hersh broke the real story.

"I knew that the CIA was operating domestically then," Ludlum says, "in fact, the book was sold to the movies and the producer - William Castle - called me and said, 'Liste, we're going to change this because everybody knows the CIA doesn't operate domestically,' I told him don't change it.

"You see," he says. "I'm not an investigative reporter but I do have to get a feel for what I'm doing. CIA agents are very easy to talk to. Get a couple martinis in them and it's not hard to jump from point A to point B. It happens all the time. They say this and you say that and blah-blah-blah.

"I remember talking to a couple of them at the time. Finally I just said, 'What you're telling me is yo're getting all this domestic intelligence information on your own.'

"One of them turned to me and said, I remember because I used this piece of dialogue. "You're f - A right because that SOB Hoover won't give us a thing."

"If a conspiracy exists," Ludlum says, "it has to be fairly imaginative. It will probably parallel something in reality. It's just not that far out of the question."

Ludlum says all this on the front porch of this three-story home in a crowded middle-class neighborhood in Leonia. N.J., a place not known for its paranoia. In fact, about the only thing Leonia is known for, Ludlum says, is that more academics live there than almost any other place in the country except possibly Boston. It is a quiet, peaceful community near the George Washingotn Bridge and the only spot in the world where Ludlum - who travels about four months a year for research and publicity - says he feels comfortable.

He is a short man with a large head and light gray hair that swirls delicately around his ears and neck. He is wearing tennis shoes, seersucker pants, a well-ironed white shirt and a gray tweed jacket almost the same color as his hair. He looks like an off-duty professor. To the neighbors, he's just Bob Ludlum, the guy who works at home.

He takes a drag on a Kent, which he smokes through a bone white cigarette holder. The only outward sign of his celebrity is a red telephone he keeps plugged in by his feet in case his agent wants to get in touch with him. He tries to explain his fictional obsession.

"I guess you could say I like a rather large canvas to work with," he says. "I take a theatrical viewpoint. It's sort of melodramatic."

Ludlum gets his sense of theater from experience. Until he was 40 years old, it was what he did for a living.

He started as an actor in 1951, after graduating from Wesleyan University, the same place he met his wife Mary Ryde, an actress herself. Acting was a short-lived career for both of them.

"My wife kept getting better reviews than I did." Ludlum says, "so naturally she had to get pregnant." They have three children - two boys and a teen-age daughter still at home.

After five years of playing parts on Broadway. Off-Broadway, regional theater and television, Ludlum was bored.

"I was out of town a lot," he says, "and getting pretty tired of acting because I didn't have any control. In 1956, I read for a television series which I didn't want to do and had to borrow money from my mother to buy my way out of the contract. I really wanted to produce. So my wife, who has a talent for the absurd - which is me - said, go ahead and try it."

He did, founding the locally well known "Playhouse in the Mall" in Paramus, N.J. "I was just dumb enough to come along when theater was expanding outside the city," he says. "I was very lucky. In fact, people say I would have started writing years ago but got interrupted by success."

Then, after about 10 years of producing, he was bored with that, too. "I found if you could set your sights low enough in the theater, you could make a killing. I had written a couple television scripts and I wanted to try writing full time. In the back of my mind I thought I might get published in a small way and then go into teaching at a university or something. I was utterly floored with my first book."

That was "Scarlatti" and it turned into one of those fantasies that keeps would-be writers punching their type-writers from California to Maine.

"We were down at the shore when I got two phone calls," Ludlum remembers. "The first one said yes, the publisher wanted the book. That was nice. Then I got another one. Not only had it been chosen as a main selection of Book of the Month but it had been sold to the paperbacks for more money than I thought existed. I must have drank up all the Scotch on the eastern seaboard. I know what it's like to be released from bondage."

The only kind of bondage Ludlum needs release from right now is his tax problems. With an income of around $500,000 a year, Ludlum is more successful than he ever dreamed.

It is, as he calls it, "utterly, utterly ridiculous." As for the money, he smiles, grins, does a self-deprecating little dance with his mouth and says, "It's like Monopoly."

As for his spectacular and relatively unobstructed rise to the very top of the suspense writing business. Ludlum defers to others: "I have a great editor. Richard Merek at Dial. I brought him 38,000 pages of sheep dip and he turned it into a novel."

His speech is littered with these little bits of self-exploding comment. He goes out of his way to credit other people for his success, even his typist. "I have a brilliant typist who sends me these scathing notes about my stuff. He is so much brighter than I am." Ludlum's editor once described his prize author as "imaginative but not too bright" and for some strange reason, Ludlum agrees - even seems to cherish the comment.

There is also a streak of honest, compulsive Puritanism in Ludlum that is as obvious in him as it is in his books. His heros are almost abnormally idealistic, with nice WASPY sounding names like Peter Chancellor. Sex is there but kept at a minimum. "I don't spend a great deal of time on things that don't move the story," he said.

That Puritanism is also the reason he's trying to sell his house in the Virgin Islands.

"I just couldn't write in the islands," he says, "and I just won't allow myself to own something I don't use. When I'm writing here, in this house, I'm very obsessive about it."

His house in Lconia is large but not particularly luxurious. It is set on a small lot bordered on two sides by a white stone walk. Inside it is a hodge-podge of rooms that run into one another or just out from the main building at odd places. A huge toy giraffe stands guard in the front hall lobby.

He works in a dark wood-paneled room separted from the kitchen by a pair of sliding glass doors. On the walls are framed copies of best seller lists - his wife puts them up - and advertisements for his books. There are some crocheted pillows with his book titles on them scattered here and there. A Xerox copier sits silently in one corner.

His desk is a heavy wooden hatch-door table in front of a couch. He gets up every morning at 4:30, feeds three cats, grinds his own coffee and then settles down to work. He writes in long hand on yellow legal pads until around 11. He takes a light lunch, reads the paper and then checks over what he's written. He copies the pages and sends them to his typist. It takes him around eight months to finish a first draft.

"I'd like to get better and better as a writer," he said, "and I hope before another decade is over. I'll write something that satisfies me. I'm writing as seriously as my talent will allow right now and I'm flattered by reviewers who are taking me more seriously than I think they should be."

And how does a man like this decide what to write about?

"Basically," he says, "I write about things that outrage me."

Apparently a great many things outrage Robert Ludlum and they all seem to come along at the right time and with the right plot.

"The Matlock Paper," for example, the story of how a group of small colleges saved themselves financially by going into the drug business with the Mafia. Ludlum says he picked up the idea originally from a newspaper article.

"I hated the fact that no one was stopping the spread of narcotics. My God, I knew of situations in some Ivy League schools where kids and parents were being blackmailed down the line for being involved in narcotics. I knew of one kid who was blackmailed by the local police for four years - it completely ruined his family."

This is perhaps one reason why his books have done so phenomenally well - there is always, he claims, a rather large kernel of truth to them all.

"I do more research for my books than my own children believe and less than I am flattered by the critics."

He is currently in the middle of a new book with another one of those nifty little three-word titles, "The Wolksschanze Covenent."

"During the end of World War II," Ludlum says, looking over what he's written that morning, "Members of the German High Command sent out hundreds of children from Germany to live in countries all over the world - the children of the damned they called them. It was their hope that some day, 30 or 40 years later, they would start the Third Reich all over again.

"In the book, they have put together this fortune - $780 million - to finance it. The hero - though he's unaware of it has - been chosen to start the wheels moving to hand out the money. I think with that kind of money, you could change the course of history.

"Now," he says, "I don't know if anything ever came of all those children sent out from Germany but it's a great hook for a novel. Suppose one of those kids is head of the Pentagon or something. I mean, my God!"