By Gary Jennings

Doubleday. 980 pp. $25

IN THORN the mannamavi, Gary Jennings -- whose previous novels include Aztec and The Journeyer -- may have found the perfect protagonist. Thorn is a "man-maiden," hermaphroditus verus, a true hermaphrodite. Fortunately the novel is told in the first person, or the choice of pronouns for the viewpoint character would have been difficult, to say the least.

Jennings attempts through this dually sexed protagonist to portray the dual nature of the culture in which the novel is set: the barbarian fringes of the Roman Empire around the turn of the 6th century A.D. Thorn's people, the Ostrogoths, were a peculiar combination of the barbarian and the civilized. The hero-king of the novel, Theodoric, was both invader and invited ruler of the Western Roman Empire. His religious faith, though Christian, was not the orthodox Catholic faith that survived into the later Middle Ages.

These themes are clearly discernible, even without the help of Jennings's several "Translator's Notes." But Raptor is not about theme or theory, or even, despite its extensive and usually well-incorporated research, about history.

This is the tale of Thorn, of all genders and none, from childhood first in a monastery and then in a nunnery, through numerous adventures across Europe and Asia, to friendship and service to Theodoric, king of the Ostrogoths. The final third of the novel, which focuses on Theodoric, is a disproportionately long, disappointingly dry recitation of events, dates and legislation. The true center of the narrative is the ever-fascinating and omnisexual Thorn in all his -- and her -- permutations.

The novel begins with the first of many sexual adventures: Thorn, then thought to be a boy, bent over a barrel by a lusty monk. Then Thorn details the events that led up to this conclusion. This structure continues for much of the book. Sexual misadventure -- the spice, as it were -- is followed by the meat of the story.

In the process Thorn travels all over northern and eastern Europe and part of Asia, and meets a worldful of characters. Notable is the enigmatic man called Wyrd, Thorn's first and most important teacher in the ways of the world and the wild. As if to balance this very masculine man, Thorn later travels with and becomes attached to the beautiful and tragic princess of the Ostrogoths, Theodoric's sister Amalamena.

In between and round about, Thorn loves and is loved by people of both sexes. Such loves always end in tragedy, in betrayal or in revenge. This is Thorn's fate, and Thorn's curse.

For Thorn, who is both man and woman, is also neither. The man can beget no children, the woman bear none. In youth Thorn tames a juika-bloth, a brown eagle, the gender of which no one ever ascertains, since it does not mate in captivity. This becomes the symbol of Thorn's nature.

Through the example of the eagle, Thorn undertakes to be a raptor, "a being uninhibited by conscience, compassion, remorse -- a being as implacably amoral as the juika-bloth and every other raptor on this earth." For "my being a mannamavi conferred one enormous benefit: that I need never have to love any other person, of whatever sex, and never have to endure all the miseries that loving entails."

Here Thorn protests too much. In the course of a long and varied career life, Thorn learns love -- sexual and otherwise. Thorn learns compassion. He also learns to hate and to kill, as far as possible, out of mercy, or need, not malice.

The true raptor, the true monster, is Thorn's alter-ego, the fellow mannamavi Thor. Thor's character and fate are a mirror image of Thorn's own. Thorn begins life as a boy, and spends much of the subsequent years -- and most of the public ones -- in male persona. Thor, on the other hand, is first perceived as a girl, and has lived primarily as one.

A HISTORIAN can argue that the novel's pervasive misogyny is a factor of the period in which it is set. But a certain balance is lacking. Jennings's female characters tend to be either tragic beauties such as Amalamena, or howling viragos.

Thor's female persona, Genovefa, is no better. And the Amazons are appalling: a tribe of outcasts and female monsters, descendants of decadent Scythians and renegade Gothic nomads. The presumption seems to be that the male is inevitably the stronger, the defender of law and right thinking, and the female, apart from the male, is both helpless and depraved.

Thorn is a memorable character, and unique outside of science fiction. A true androgyne, however, would be convincing as both male and female. Thorn is essentially a man with female equipment.

As a truly universal protagonist, Thorn falls short of the mark. And, as a novel, Raptor might better have ended with Theodoric's taking of Rome, and a second complete novel spun off from the events thereafter.

Yet these are, in the end, minor reservations. Raptor is a splendid entertainment: a historical novel of the old school, impressively researched and remarkably accurate -- and above all, a roaring good read.

Judith Tarr teaches at Wesleyan University in Connecticut. Her new novel, "Lord of the Two Lands," a novel about Alexander the Great, is forthcoming.