HERE BE DRAGONS. By Sharon Kay Penman. Holt Rinehart Winston. 704 pp. $19.95.

MEDIEVAL MAPMAKERS had a handy way of dispensing with the unknown. After the mountains were charted, the lakes and rivers lettered, the recognized borders carefully drawn, the cartographers found themselves face to face with the land, seas, the void, whatever, that lay beyond their ordered realm, their inked-in lines. So on those mysterious and vast white spaces the mapmakers would letter the words "Here be dragons." And who but the very adventurous would seek to prove them wrong?

It is not likely that Sharon Kay Penman, in constructing all 704 pages of her second historical novel, Here Be Dragons, ever found herself wondering what to do with white space. If in her retelling of the history of England's Plantagenets, their Angevin empire, the strife with France and with renegade Wales, she let slip through the cracks a single border war or a character of minor historical importance, then let the more adventurous, the pursuers of 13th- century trivia, find them and seize on them. For the rest, there is everything in Here Be Dragons but dragons: princesses held captive in stone towers, bloody wars, princes scheming to dethrone their own brothers, castles under siege, maidens in distress, power struggles for half of civilization, rampant infidelity (personal and professional; mistresses abound and a treaty isn't worth the parchment that it's written on) lusting, mead guzzling, wine drinking, love affairs that topple kingdoms -- how did England survive the 13th century?

The villain and hero of this sometimes imperfect amalgam of history and fiction is King John, a man heir not only to vast realms but also an extraordinary gene pool; his father, Henry Plantagene, was duke of Normandy, then king of England; his mother, Eleanor of Aquitaine, was queen of France at age 15, and then queen of England, abandoning the boring French Louis for the handsome Henry.

John, as their fourth and youngest son, is stuck in a slot that destines him more to some comfortable manor than to the throne. But the kingly competition keeps eliminating siblings, one brother lost to "bloody flux," one to tournament injury and one to a wound suffered in a minor skirmish. History serves John well; he seizes control, and when a nephew, Arthur, starts to rally support against John, the young prince is arrested and then "disappears," his body never found, his untimely end forever the subject of speculation. Others of John's enemies starve to death, are hanged or live to see their lands seized and their sons held hostage.

BIG BAD JOHN makes a great villain, but he was also, like other legendary beasts of history, an intelligent and caring ruler. Penman shows this side of John, too, through the perspective of his illegimate daughter, Joanna, a young girl cursed with her father's dark and brooding looks and with the knowledge that the same man who slaughtered a 7-year-old also took her in, cared for her, loved her.

Joanna's character serves other purposes. Her marriage to the handsome Welshman, Llewelyn, has been arranged by her father, a politically expedient maneuver that will keep the Welsh under John's thumb. The union may keep swords from clanging for a while, but the cultures clash all the louder. Joanna gets caught between her father and her husband, between England and Wales, between son and stepson. These conflicts are of historical interest, of course, and no doubt represent the unfortunate mess that was often the result of such prearranged marriages common in that day.

But the Joanna-Llewelyn match turns into something else; far from being a cold, dynastic arrangement, the marriage is madly, tempestuously passionate. And Joanna, sometimes walking around in thin chemises or towel-drying her hair, has a dangerous way of driving the author away from sensitive, sensible prose and precariously toward romance novel jabber (as in "throbbing, languid warmth"). Since she's married to a Welshman, the 13th century's equivalent of a liberated male, she's allowed to be more of a personality than her English counterparts, but she shouldn't be allowed to wander off so far that she ends up acting like a character played by Jane Seymour in a mini-series.

History and fiction bound up together in historical novels have always had their own uneasy alliance, a relationship as skittish as any of King John's treaties of convenience. Penman deftly makes the mesh work; legions of characters are introduced, and each has personality; catapults don't lumber across a field just to show themselves off; even John's favorite dish of stewed lamprey eels with saffron sauce goes down smoothly.

But which is -- or should be -- driving the car? Is history a passenger picked up to give importance and direction to an otherwise colorful but purposeless trip? Or is history in control, subplots, intrigues, imagined intimacies being supplied by fiction only to embellish a road map that rules all?

Here Be Dragons changes drivers. Of its 704 pages, history rules the first 200, relentlessly yet vividly introducing characters by the realmful -- Norman, Saxon, English, Welsh, all their families, advisers, bastards and ever overlapping relationships. Then fiction has its turn, taking the outline and plumping it up with flesh and color, flushes on cheeks, explosive tempers, thundering hooves, the intimacies of the bedroom. But then the book can abruptly cut back to business, take an unfortunate timeout for historical tallying: who's dead, who's on crusade, who's allied with whom, who's got which castle. It's the result, obviously, of Penman's tenacious and plentiful research -- the dates, the places, the times, the details well noted. But the burden of this diligence falls on the reader, and leads the mind to wander to the margins of the pages, to those thin strips of white space, with the hope that somewhere on those uncharted spaces a dragon lurks.