THE UNTOUCHABLE.<i> By John Banville</i> .<i> Alfred A. Knopf: 354 pp., $24</i>

In “The Untouchable,” the prolific Irish novelist John Banville works his characteristic disquieting pattern into the longest and most subverting British spy scandal of the century. Banville’s is a pattern of moral ambiguity, of hermetic mystery peeled away in adhesive layers and of an artistic conscience as rich and crawling as old Stilton cheese.

Here the corrupt conscience belongs to the narrator, Sir Victor Maskell, fictional alter ego to the late Sir Anthony Blunt. Like Blunt, Maskell is publicly revealed in the ‘70s as a former Soviet spy and fired from his posts as director of the Courtauld, Britain’s preeminent art institute, and as director of the queen’s art collection and stripped of his knighthood.

Blunt was labeled by the press as the Fourth Man. Man One and Man Two had been Guy Burgess and Donald McLean, British diplomats and intelligence operatives who escaped to the Soviet Union in the 1950s after being warned of imminent arrest. The person who warned them, headlined as the Third Man, turned out to be Kim Philby, a top intelligence official who fled to Moscow a few years later.

Blunt was known to the authorities as another member of the ring--mostly former Cambridge intellectuals recruited at the university. Several, including Blunt, were homosexuals, which in Britain was then popularly considered almost as bad as being a spy.

Yet for more than 20 years, though the authorities repeatedly interrogated him, they kept his identity secret and he continued to exercise his profession and knighthood. After his unmasking, the press shifted to seek out a Fifth Man. A Sixth Man was rumored.

The names of several high officials and politicians were bruited about, though no further identifications were announced. Cover-ups, shameful loyalties and old-boy indulgences were alleged; it was the demoralization of a slice of the ruling class and, since that class felt it embodied society, of society as well. Besides Blunt, Banville uses Burgess and McLean, their names barely disguised (all three are dead, as is Philby, whom he does not use). He makes his Sixth Man, a top Conservative minister, “The Untouchable” of the title--a fictional device that serves to end the novel on a high paranoid note: They’re still here.

The Fifth Man is a shocker. Banville makes him Graham Greene, dead now and unable to sue. Except for his name, Querrell, he is not even thinly disguised--other than as a Soviet agent. He is the book’s most loathsome character. It is hard to know what Banville’s animus could be, unless it is that Greene was a much better writer and wrote a better book (“The Human Factor”) on some of the same events.

This is not to say that there are not some very good things in “The Untouchable.” As Maskell tells his story to a woman who poses as a writer--she is something more shadowy--he can be sharp and penetrating and, once in a while, moving. He begins a day or two after his downfall and, despite or perhaps because of the arrogant snobbery of his voice, we sense the pain. A man of poses and disguises all his life, he feels the loss of the knighthood even more than that of his high place in the arts. His posture was his essence.

Wandering back and forth from past to present, Maskell evokes a whole decadent Bohemia of upper-class British intellectuals and dissipaters in the ‘30s. Champagne, sex, snobbery: It is a world of the early Evelyn Waugh and the novel-cycle of Anthony Powell. That is the trouble: We have been here before and to much more effect.

At Cambridge, a dilettante Marxist, he and several of his friends are recruited by a Hungarian fur dealer whose business masks his work as a Soviet agent. One of the best moments in the book is the series of silences and roundabouts as they bask in deck chairs on the college quadrangle, followed by a half-spoken but unmistakable offer and acceptance.

“The phenomenon was strange to me still but would become increasingly familiar the deeper I penetrated into the secret world. It marks that moment when a group of initiates, in the midst of the usual prattle, begin to go to work on a potential recruit.

“It was always the same: the pause, the brief tumescence in the air, then the smooth resumption of whatever the subject was, though all, even the target, were aware that in fact the subject had been irretrievably changed. . . . Nothing so tentative, nothing so thrilling, excepting, of course, certain maneuvers in the sexual chase.”

Later, Maskell will compare his role as a spy to his secretive nocturnal pursuit of male partners. Despite romantic crushes--one of which puts him under the sway of the man who will bring about his public downfall--he discovers his homosexuality only in his 20s, after the breakdown of his marriage.

Maskell tells of his father, a kind and upright bishop in Northern Ireland, and of his exuberantly affectionate stepmother. He recounts various exploits for British Intelligence, under cover of which he was spying for the Russians. Shame gets between him and even his best memories; he is unable to explain his spying other than as a need for purpose and discipline and for the sense of omnipotence that clandestineness bestowed.

He has come to doubt even his lifelong devotion to art and beauty. After the betrayals he has committed and those that have been committed against him (his two children may not be his, and the decision to publicly disgrace him, after all these years, was an act of cold expediency), it seems to him that even his scholarship has been a disguise.

He sends his most beloved possession, a Poussin painting, to be cleaned and valued, and suddenly he is afraid. Sure enough, as part of the abasing denouement, he hears that it may be a fake.

Maskell’s voice is too narrow for any but the book’s narrowest purposes: recrimination and a brutal account of how the world works. There is little but rage and self-hatred in it. He tries to summon up better things--a few golden moments in his youth, a glowing friendship, a momentary sweetness at the start of his marriage--but very little gets through.

Maskell trying to convey the notion of grace lost is like a tortoise trying to convey the notion of speed lost. His voice is the book. Too thin to establish a larger reality against which its raging fall can play itself out, its failure is also the book’s.