All the Stage Is a World : Medieval murder-mystery questions, explores capacity of theater to shape lives : MORALITY PLAY, <i> by Barry Unsworth (Nan A. Talese/Doubleday: $22.50; 192 pp.)</i>

It is three years since Barry Unsworth’s last novel, “Sacred HunHger,” won plaudits and prizes (including the United Kingdom’s prestigious Booker Prize) for its rich, harrowing portrayal of lives aboard an 18th-Century English slaving-ship.

The setting of his new book is very different, and the tone of it even more so. “Morality Play” tells the story of a troupe of players on the road in late 14th-Century England. The action unfolds over a few days, and features a tight ensemble of characters. Most of it takes place in a small, unnamed Yorkshire town where the actors arrive, in the deep midwinter, and set up their stage in the inn-yard. After the epic sweep of “Sacred Hunger,” this is a spare and sharply focused piece. It has the deceptive conciseness of a parable, or indeed a medieval morality play, in which the complexity lies not in the telling of the story, but in the meanings and resonance that echo in the mind after it is told.

If this makes it sound rather austere, I should add straightaway that it is also an intriguing murder-mystery, which keeps you guessing until almost the last page.

The story is narrated by a young man called Nicholas Barber. “A poor scholar, open-breeched to the winds of heaven,” he has run away from his tedious studies as a sub-deacon at Lincoln Cathedral, and taken to the open road. Cold and hungry and full of remorse, he falls in with a troupe of strolling players. They have recently lost one of their members, the comic Brendan; Nicholas is taken on as a replacement; “it was a death that began it all and another death that led us on.”

The players, led by the pale, charismatic Martin, are en route for Durham, to entertain at a nobleman’s Christmas. Somewhere in Yorkshire they stop off to bury Brendan, and to eke out a few shillings with a performance of the “Play of Adam.” Nicholas makes his debut--as an attendant demon with a horned mask and a rope tail, and a trident “for roasting the damned”--and learns the dangerous excitement of the stage: “A mask confers the terror of freedom, it is very easy to forget who you are. I felt it now, this slipping of the soul.”

Unsworth handles the playing scenes with quiet relish: the repertoire of gestures and symbolic costumes; the drums and the torchlight and the alarming masks; the appearance of God on stilts. He catches that vividly makeshift quality of early drama.

Then the story begins to darken. They learn of the recent murder of a 12-year-old boy, Thomas Wells. A young woman, a weaver’s daughter, has been charged and--rather hastily, it seems--condemned to hang. The circumstances are shady, and become more complex as the players start to inquire about them.

There is a key scene, beautifully written, in which Martin broaches the idea that they should perform a play about the murder. To the others, this idea of representing a real event, rather than the stock themes and figures of the moralities, is extraordinary. They are nonplussed. “Who plays things that are done in the world?” says one. “It was finished when it was done. How can men play a thing that is only done once? Where are the words for it?”

As their “Play of Thomas Wells” becomes, in itself, a form of detection and inquiry, and as the finger of suspicion points ever higher toward the household of the local feudal magnate, Richard de Guise, one sees that Unsworth’s story is about the capacity of art--and particularly the live, momentary art of theater--to create new meanings, and thereby new possibilities, in the lives of its audience.

Their performance is contrasted with the chivalric jousts and tourneys that also feature in the story. In these the knights and ladies play their parts in a performance that reinforces the hierarchies and assumptions of feudal society. The play does something different: It questions and explores, and within the marked-out boundaries of the stage, and with the active collusion of its audience, creates an area of comment and debate.

As a medieval murder mystery, “Morality Play” invites comparison with Eco’s “Name of the Rose,” and to admirers of the latter it will perhaps seem somewhat skimpy. I personally find its cool, lapidary style something of a relief. This is not a historical novel that depends on texture--that brothy accumulation of period detail, picturesque squalor and archaic turns of phrase--to achieve a sense of the past. Even the best practitioners, like the late Anthony Burgess, tend toward this rhetorical construction of history. Too often a historical novel seems like a piece of repro furniture deliberately “distressed” to make it look like an antique.

Unsworth’s story wears its authenticity lightly, and his dialogue is entirely free of “gadzookery.” The social setting is cleverly evoked: wintry scenes, hard-bitten lives etched against a background of frost and snow. There is a sense of confinement and control--the presence of hunger and plague; the daily oppression of feudal society; “Morality Play” is a book of subtlety, compassion and skill, and confirms Barry Unsworth’s position as a master craftsman of contemporary British fiction.