The Bard's contemporaries will get a hearing in new Swan theater

''It's a fairy tale,'' says Trevor Nunn, artistic director of Britain's world-renowned theatrical troupe, the Royal Shakespeare Company. ''And it's just wonderfully invigorating to know that fairy tales actually can come true.''

Mr. Nunn is referring to the recent announcement that the RSC will soon be opening a unique new theater at William Shakespeare's birthplace, Stratford-upon-Avon - an event being made possible by the generous gift of a chance visitor to this picturesque Tudor town.

The only stipulation attached to the unexpected bounty is that the benefactor's identity, including sex and nationality, must remain anonymous. Even so, there's strong speculation he/she is an American. But Nunn is giving nothing away. ''I think the person comes from the moon,'' he quips.

Faithfully reconstructed in Jacobean style and using many of the original staging techniques, the new theater, to be called The Swan, will be dedicated entirely to the hundreds of English plays written by Shakespeare's ''contemporaries'' - a period which, Nunn says, extends from roughly 1570 to 1750.

Nowhere in Britain or beyond is there a place devoted to such an ambitious challenge: Most of these plays, until now generally considered too difficult to stage effectively, have not been performed since the time of their creation. Indeed, the significance for the RSC of The Swan - appropriately named after the 17th-century playhouse set up to compete with Shakespeare's Globe Theater - cannot be overstated.

''I think the new theater is the most important development for the RSC since the company (which began in Stratford over a century ago) established a second base, in London (1962), and started doing modern plays as well as Shakespeare,'' says Sir Peter Hall, former head of the RSC and now artistic director of Britain's other flagship drama company, the National Theater. ''There's a huge repertory of neglected plays of great worth, some 200 years of drama, which can teach us what Shakespeare came out of and what he contributed to. Yet they cannot be done in the normal circumstances of the theater. Stratford is the natural place to do them.''

The Swan is scheduled to open in early 1986, and Nunn, along with the rest of the RSC, is still reeling from how it all came about.

As he explains it, the idea has actually been around for some time, but until now was little more than a pipe dream. Despite the fact that the RSC is probably the most famous drama company in the English-speaking world and has dazzled audiences on both sides of the Atlantic with such recent productions as ''Nicholas Nickleby,'' ''Good,'' and Terry Hands's ''Much Ado About Nothing,'' it's been impossible to come up with the necessary cash to get the project off the ground; in the face of Britain's longstanding theatrical recession it was, however worthy the idea, a luxury that simply couldn't be afforded.

Then one rainy day in Stratford ... ''this visitor, this person,'' continues Nunn with irrepressible enthusiasm, ''went into the art gallery here, simply to get out of the rain, and there saw a model of the proposed theater. It was in the gallery because we had long ago given up hope of it ever becoming a reality.''

Mild curiosity and unceasing rain prompted the visitor to ask the curator about the model; growing fascination eventually led to a chat with Nunn himself. After going over the plans in detail, ''this person fell in love with them,'' recalls Nunn, and immediately wanted to foot the bill - said to be well over $3 million - out of private funds.

''I thought it was a practical joke,'' he laughs. ''I was expecting to hear for the next six months after the event that it was all a hoax.''

But it wasn't. And the general feeling in this country is that The Swan will make a significant contribution not only to the RSC, but to British theater generally. A key function of the RSC, according to its founding charter, is to keep alive the works of Shakespeare. Even though the company is the acknowledged world leader, there has been a growing attitude within the group that it could do its job better.

''Our audiences don't know enough about Shakespeare's period, and neither do we,'' admits Nunn. ''We make misjudgments about his work because we don't have enough to go on.'' With a theater devoted solely to studying this important era, he maintains, that neglected gap can begin to be filled.

Indeed, it has long been advocated in this country that the full range of British dramatic tradition is not being represented in today's theater and that the RSC, therefore - in keeping with its role as a national institution - should help lead the way into the uncharted areas.

''There are only 37 plays by Shakespeare,'' observes Irving Wardle, theater critic for The Times (London), ''and they come round and round like a merry-go-round here, almost to the point of overexposure. So to have a new theater devoted to exploring this other, largely unknown, period is a great contribution and very much needed.''

The arrival of The Swan also means the RSC can turn more of its multi-talents to modern drama. The company at the moment has in Stratford the 1,500-seat Royal Shakespeare Theater and another much smaller stage nearby, rather cheekily called The Other Place, plus two comparable performing spaces in London. The two larger theaters are used for the main Shakespeare repertoire, while the smaller ones tend to be for the potentially less popular ventures: experimental treatments of Shakespeare's plays, period pieces and, when it can be fitted in, the new or neglected work of modern playwrights. With the additional stage, it will give the smaller RSC theaters greater opportunity to expand this latter repertoire. ''It simply makes all things a bit more possible,'' says Nunn.

Perhaps most important, though, The Swan is viewed as a godsend for preserving Stratford as the world center for Shakespearean studies. Although the town attracts streams of visitors every year solely for its famous connection, Nunn sees television, cable, video, and a revived movie industry as very real threats to what Stratford has to offer: The RSC can no longer take its audiences for granted.

''We want to have much more discussion and education of Shakespeare based in this theater,'' says Nunn. ''Stratford could become an abandoned place. We have to fight for it.''

But the big question is: Will audiences be lured in large enough numbers to see the kind of high-minded theater fare The Swan will be offering?

The general consensus among observers here is that the RSC knows how to succeed where perhaps others might fail: ''Our first consideration will be to make these plays come alive,'' insists Nunn. ''We certainly don't want to open a theater in order to prove to everyone that the plays of Shakespeare's contemporaries are a load of academic boredom. On the contrary. We want to give Shakespeare more of a run for his money.''