A MAGGOT. By John Fowles. Little, Brown. 455 pp. $19.95.

SOMETIMES I remember an opening lecture in a class in critical writing at the University of Missouri School of Journalism. The professor, a no-nonsense type from newspapering and World War II, told us how you make a review.

It wasn't so hard, he said quietly. First you decide what a writer or composer or painter is trying to do in a work. Next you determine whether he accomplishes it. Finally, you say whether it was worth doing in the first place.

My mind drifted back there while wrestling with what to make of John Fowles' A Maggot. It isn't so easy. For Fowles is a purposeful writer who counts even though A Maggot may not. His The French Lieutenant's Woman showed us what existentialist attitude and cinematic literary technique can yield from one of the oldest plots in the world, a woman abandoned by her lover. It's probably, as Anthony Burgess pronounced it, one of the best novels published in English since 1939. With The Magus, The Collector, and other works Fowles has extended the myth-making of fiction into new territory -- and with unusual commercial success.

Fowles has been as good for fiction as fiction has been good to him. Fiercely protecting his personal isolation and professional integrity, Fowles has been a writer with a mission -- to make the novel live anew by becoming alive for him. Indeed, he told the English writer John Boston several years ago that he doesn't need an audience to write for. He would go on writing if he were the only person in the world.

Which is probably why I remembered those lectures of the late 1940s. John Fowles has done what he intended -- to please himself by making an historical novel out of a whim. But by almost every othermeasurement A Maggot is a disappointment. It's a prolonged mystery that remains a mystery. It's a morality tale propelled by an occult conversion to an aberrant form of Christianity. Its original protagonist remains nameless and physically disappears after the first 60 pages, replaced by an ill-tempered lawyer conducting an investigation. Fowles' linkage of feminism and dissent has a parallel in the present, but basing the connection on sorcery or subterfuge robs this observation of its pertinence.

Fowles begins his novel with a prologue explaining that "an older though now obsolete sense of the word (maggot) is that of a whim or quirk. . . . For some years before its (the novel's) writing a small group of travellers, faceless, without apparent motive, went in my mind towards an event. Evidently in some past, since they rode horses, and in a deserted landscape . . . The riders never progressed to any destination, but simply rode along a skyline, like a sequence of looped film in a movie projector; or like a single line of verse, the last remnant of a lost myth.

"What follows may seem like a historical novel; but it is not. It is a maggot." (Prologue)

The myth Fowles fashions from this quirk takes place between April and November of 1736 in the southwestern counties of England near the Bristol Channel. It is peopled by his five travelers -- four men and a woman, none of whom is as first appears -- plus the inquiring lawyer, an innkeeper, maid, minister and others who met or saw the travelers before horror befell them. One of the men was found hanged. Another simply vanished. The lawyer's task is to find out where this band of travelers was going and why, and especially what happened to the disappeared leader of the group, an imperious and moody young aristocrat in his twenties.

Gradually, through events and sworn testimony, we learn that the young aristocrat known as "Mr. Bartholomew" had hired (except for his deaf-and-dumb manservant) the others for the trip -- a middle-aged Haymarket actor, an alcoholic Drury Lane doorman, a spirited "public whore" from a London "bagnio." Also traveling incognito and under false names, they were given different explanations for the journey. Bartholomew told the actor Lacy and doorman Jones they went as his protectors against those who would prevent his reaching a young woman near Amesbury with whom he was "in love and half dead of it"; to them he passed off the prostitute as her future maid. Bartholomew told the whore Fanny she was hired to perform sexual acts with his manservant Dick safely away from London so that he, Bartholomew, might watch and overcome his impotence. Fanny's employer, when deposed by the lawyer Henry Ayscough, says Bartholomew told her he was taking Fanny to a gentlemen's party near Oxfordshire where a prize would be awarded for bringing the most adroit trollop.

THE ONLY TRUTH is that Fanny and Dick perform as planned -- and more. Bartholomew has in fact undertaken the journey to induct Fanny, with himself and Dick as acolytes, into the nether world of sorcery and witchcraft or a transcendental state of divine communion and futuristic vision. Both are reported as having occurred.

Jones, who later says he witnessed some of what happened in the moorland cave on May Day, believes Fanny was led into sexual intercourse with the devil. Fanny, later known as Rebecca Lee, says she was redeemed in the cave from her life of sin by a young carpenter, an old bearded man, and a silver-sheathed woman who took her up a maggot-shaped balloon (or spaceship) and showed her the earth as "June Eternal."

Ayscough uncovers more that's mad, miraculous, or inexplicable. Pregnant with poor Dick's child (he was found hanging with violets stuffed in his mouth), Fanny/Rebecca has converted to a splinter Anabaptist group, rails against women as men's property and decries the brutal British social order. She denies the devil, swears to futurist visions of "June Eternal," and sees ghosts. She has married a self-pronounced prophet in Bristol.

As with The French Lieutenant's Woman, John Fowles designs A Maggot to draw his reader into imaginative alliance with him through technical devices. He interweaves his narrative with pages of The Gentleman's Magazine from 1736. He enters the novel as modern authority on the non-status of women and the oppressive circumstances of England's poor and powerless under George II. To intensify the illusion of lived lives, he contrasts the probable attitudes of his characters, were they alive now, with their mindset in 18th-century England. And in a bravura exercise of literary temerity or ingenuity Fowles announces in a lengthy afterward that Rebecca's child was the historical Anna Lee, founder of the Shakers, who preached social unorthodoxy in England and America and believed the Second Coming would be feminine.

But Fowles can't or won't show us chracters changing, mysteries unlocked, or plots concluded. He keeps the instructive pleasures of fiction to himself. He has written two narratives -- one about a death, disappearance, and an occult rite we're to explain since he doesn't; the other about a radical personality alteration (from atheist whore to proselytizing Christian) we're supposed to credit but only Fowles has witnessed.

Fiction pushing the frontiers of the form? No, not this time. Instead, an intensely narrow insight into the social mechanisms of pre- industrial revolution England, a lecture on the courage of dissent, and intellectual self-indulgence posing as fiction. As promised, we observe a maggot. It worms itself into John Fowles' mind. It goes no further distance.