THE BOURNE SUPREMACY. By Robert Ludlum. Random House. 597 pp. $19.95.

SLOGGING THROUGH The Bourne Supremacy, I found myself considering this question: which is more ridiculous, the dialogue or the plot? Here (not quite picked at random, but I didn't have to turn very many pages to find it, either) is a sample of the former:

"'A commando.' said Jason quietly. 'It fits. Who is he?'

'He's a man without a name but not without a macabre story,' replied d'Anjou, gazing at the mountains in the distance.

'No name . . . ?'

'None he ever gave me that he would not contradict in the next breath -- none remotely authentic. He guards that name as if it were the sole extension of his life, its revelation inevitably leading to his death. . . . No mere foot soldier he, Delta. He was a captain at twenty-two and a major at twenty-four when rank was next to impossible to obtain due to Whitehall's service economies. No doubt he'd be a brigadier or even a full general by now if his luck had held out.'

'That's what he told you?'

'In periodic drunken rages when ugly truths would surface -- but never his name. They usually occurred once or twice a month, several days at a time when he'd block out his life in a drunken sea of self- loathing . . . '"

And so on. That particular conversation (between two men who have just come through a bloody firefight, incidentally) runs on in the same vein for four or five more pages; equally improbable passages of dialogue can be found on almost any page of the book.

As for Robert Ludlum's plot, it defies brief description, but involves a former American diplomat, now a scholar of Oriental studies, named David Webb. Webb has a second personality, however: a Rambo-like super-guerrilla called Jason Bourne, also known as Delta.

Bourne, we discover, was created and manipulated by cynical Washington officials to carry out sinister American activities first in the Vietnam war and later in an unsuccessful attempt to trap the celebrated international terrorist Carlos.

As The Bourne Supremacy opens, those same officials are planning to reactivate Bourne for another mission -- the assassination of a fanatical Chinese conspirator (ostensibly a high-ranking member of the Peking government, but with his real loyalties elsewhere) who, if he is not killed, threatens to plunge Asia and the rest of the world into chaos and war.

TO FORCE AN unwilling Webb to become Bourne again and carry out their plans, his manipulators kidnap his wife, Marie, and whisk her off to Hong Kong. Webb, transformed into Bourne, jets off after her and spends the next 500-plus pages dispatching his enemies with bullet, knife or garrote, strewing their corpses around suitably exotic Asian locales while plot convolutions far too complicated to be summarized here unfold in one preposterous scene after another.

When it's all over, naturally, the world is saved from catastrophe, Marie (a bit of a super-heroine in her own right) is reunited with her husband, and he is David Webb again, free of the violent identity of Jason Bourne forever -- or will it be only until Bourne is needed for a sequel?

Beyond this, it's difficult to know what a reviewer can pertinently or usefully say about this novel.

Those readers who have made commercial successes of Ludlum's previous novels will no doubt find this one equally to their taste. Its appeal lies, presumably, in the very absurdity of plot and characterization that will turn off those who don't care for the genre. In the last few pages of the book, Ludlum has one of his characters say, "These people do things the rest of us only dream about, or fantasize, or watch on a screen, disbelieving every moment because it's so outrageously implausible." It's a remark that may also help explain why this novel will undoubtedly join its predecessors up in the best-selling stratosphere.

If The Bourne Supremacy delivers exactly what Ludlum's fans want and expect, should it matter if a non-fan finds it bloated, witless and boring? Probably not. There's one point that should be made, though, and it has to do with Ludlum's portrayal of China and the Chinese. This is a book that depends on all the crudest stereotypes of the sinister Oriental. Whenever Chinese appear in these pages they are shown as diabolical, sadistic and depraved, practicing villainy as a sort of religious cult.

These have been staple images in Western popular fiction for a long time, of course, but they can still wound, and they grossly distort our view of a rich, ancient and complex civilization. To the extent that The Bourne Supremacy perpetuates that distortion, it is entertainment that is not quite harmless after all.