AMADEUS, by Peter Shaffer; directed by Peter Hall; designed by John Bury; with Ian McKellen, Tim Curry, Jane Seymour, Gordon Gould, Paul Harding, Patrick Hines, Nicholas Kepros, Louis Turenne and Edward Zang.

At the National Theatre through Dec. 6.

A visit to "Amadeus" is like a merry ride around the beltway of an Oz-like city, with the delights of downtown towering seductively (but unreachably) on the horizon.

At first, this elaborate historical fantasy beckons us toward such wonderful thematic matter as genius, talent, celebrity and envy, lighting the territory in a blaze of rhetorical, scenic and thespian razzle-dazzle. Then playwright Peter Shaffer heads off toward a hokey, puzzle-play finish that is an insult to history and human nature.

"Amadeus" is based on the short, under-appreciated career of Wolfgang Amadeus Mozart and the long, over-appreciated career of Antonio Salieri, court composer to Emperor Joseph II of Austria. Posterity has turned the tables on these two, of course, and Shaffer supposes that Salieri always knew it would -- knew, in fact, almost from the moment of hearing Mozart's music and then meeting the vulgar young man from whom it had, incredibly, issued.

As Shaffer pictures him, Salieri was a no-talent bum with just enough know-how to appreciate, and bitterly resent, Mozart's genius. He alone, ironically, understood what Mozart was doing when the trend-setters viewed him as an undisciplined prodigy, a man of "too many notes." (In the play, the emperor is given to yawning during Mozart's concerts, and pleasantly informing the composer that his work is "coming along nicely." But "I really think that we must omit encores in future," he adds after one performance. "It really makes things far too long.")

The relationship between the two composers -- a relationship acted out, mostly, inside Salieri's head -- forms the story of Shaffer's play, first performed at London's National Theatre and now mounted anew, in grand style, for Washington's theater of the same name. The theme of the play parallels Voltaire's observation that "the best is the enemy of the good." Mere talent, in other words, cannot coexist comfortably with genius.

It is Salieri himself, half-mad and (he thinks) at the end of his life, who escorts us -- in flashback -- through the years when he and Mozart inhabited Vienna. He had arrived there first, and he had "arrived" in the professional sense too. As a music-smitten 16-year-old, he tells us, he had tried to strike this bargain with God: "Signore, let me be a composer! Grant me sufficient fame to enjoy it. In return I will live with virtue. I will strive to better the lot of my fellows. And I will honor You with music all the days of my life." And God had seemed, for a while, to uphold His end of the bargain. In Vienna, Salieri had fame, riches and the best possible connections. Then along came Mozart -- a "voice of God" coming from an "obscene child." Shaffer takes great pleasure in shattering the conventional, porcelain image of Mozart, substituting a foul-mouthed, gleeful young lout. And he has imagined the preliminary skirmishes between Mozart and the civilized Salieri with gusto -- which is just how Ian McKellen and Tim Curry play them.

As Salieri, McKellen acts from the head and the lungs, and he has a marvelously sour and pliable face. His character's chief indulgence, appropriately, is an addiction to sweets. Curry, on the other hand, acts from the feet and the behind, and takes wonderful ecstatic, mincing steps. He is continually climbing all over the furniture -- and all over various young women (his chief indulgence), perhaps not fully appreciating where the furniture leaves off and they begin.

Jane Seymour has enough stage presence to make up for a less vividly drawn role as Mozart's wife. And her big scene -- pleading for her husband to an influential, older, secretly lecherous man -- is surprisingly fresh and funny, especially if you stop to consider how many times playwrights have imagined this encounter -- or close variations thereon -- down through the centuries.

All this is the first act, a crafty and energetic one that sends the audience brimming into intermission (on what has to be an original pretext too -- the narrator's weak bladder). But in the second act, the vigor of McKellen's and Curry's performances begins to wear off, and the thinness of their characters becomes more evident. Shaffer's picture of Mozart never gets far enough beyond the idea of genius-as-boor. We never even begin to understand where these two qualities spring from, and just how they meet in Mozart's personality.

Salieri is more lamentably incomplete. Shaffer would have us believe that Salieri is riding along smoothly, entirely content with himself and his accomplishments until a genius invades his world and exposes his worthlessness. aFrom that point forward, he becomes convinced (as Shaffer seems to be) that he has no talent whatsoever, and he yearns for Mozart's utter ruin. Unfortunately (or fortunately), that is not how humans work. Confidence is something that ebbs and flows, and so -- perhaps inversely -- does envy.

It is a safe bet that a musician whose pupils included Beethoven, Liszt and Schubert had some talent and some awareness of it.To have been affected by Mozart in the way Shaffer suggests, Salieri would have to have been an insecure man to begin with. Even so, to be human he would have to have had moments of conscience, moments when he wanted to acknowledge Mozart's greatness.

Such tensions are the missing ingredient of "Amadeus," and especially of its laborious second act. Instead, Shaffer has stuck doggedly to the question (based on historical rumor) of whether Salieri may have murdered Mozart. He has strung out a great deal of melodramatic folderol that leads to a ponderous final confrontation between the two men aimed at answering that question, with an appropriate dose of ambiguity.

It should be noted that "Amadeus" bears a conspicuous resemblance to Bernard Shaw's "The Doctor's Dilemma," another play about a boorish genius, his devoted wife and "a little man who tried to kill a great one."