THE National's "Amadeus" is the most masterful theater our town has seen in too many years.

Brilliant theatricality is the essence of Peter Shaffer's play and its lavishly exciting production, from the superbly mercurial performance of Ian McKellen to the elegant designs of John Bury.

Bury's curtainless stage picture asserts what is to come: A sloping, gleaming blue floor -- at its rear a graceful -- archway -- is topped by guilded baroque angels. There the scene changes will take place in the seamless style Shaffer has chosen to cover 40-odd years of Viennese intrigues.

In view at center rear as you take your seat is a wheelchair, its back to the audience. Somehow, you don't quite notice how, there appears a figure in that chair, its capped, baldish head just visible over the chair's back. Imperceptibly, the house lights go down and we meet the play's central figure. i

This is Antonio Salieri in old age, leading us into events that began 42 years before, when he was Vienna's most admired composer and just beginning to hear about the young man, six years his junior, who would threaten his supremacy: Wolfgang Amadeus Mozart.

The basis of Shaffer's play is the legend that Salieri poisoned Mozart. But Shaffer is not about to show us another Claudius dipping the pearl of poison into some Hamlet's cup wine. What he does show us, over the years, is how Salieri constantly strove to cut down his rival through inference, innuendo and inaction. This, Shaffer demonstrates, is as poisonous as hemlock. There will be 34 years between Mozart's death and Salieri's decision to slit his own throat -- which will not, after all, cause his death.

As he has in all his plays ("Five Finger Exercise," "The Royal Hunt of the Sun," "Black Comedy" and "Equus"), Shaffer deals with irony. For him, history can be an all-too-fallible record. He is a questioner, never pretending to tell us what happened, carefully setting forth options, choices, possibilites, those irritating requirements of the aware life.

Nor does Shaffer show Salieri as a despicable, no-talent climber. His work is acceptable to his particular audience, he works hard at it and at self-promotion, a human craft that has brought him rank at the imperial court of Joseph II.

Shaffer draws us into a period when men believed in God -- a comfort to the luckless and a surcease to the fortunate. Salieri tells us how he long ago made a pact with God: He would serve Him in all his commandments in exchange for the favor of being the most famous musician of his time. That, Salieri comes to realize, is exactly what God does grant him, fame and fortune through his music. But, believing in God as he and his contemporaries do, Salieri realizes that he did not ask for the rarer gift of genius. Mozart was born with it.

Shaffer's Mozart is another twist, another questioning of history's unreliability. Instead of showing the pretty figure custom has created, Shaffer's Mozart is a cocky, spoiled, messy child, a symbol of unsullied genius: "touch it and the bloom is gone."

In old age Salieri continues his war with God. He recognized that his intended rape of Mozart's wife was avoided not through her refusal but through the self-disgust God had forced into his conscious mind. Trying to revive the early gossip that he had poisoned Mozart, Salieri comes to his personal paradox. Proclaimed as innocent, he alone will know his true guilt: God's strict, final taunt.

All this -- and far more -- is achieved through marvelous theatricality. Words or incidents inspire the music we hear gently in the background, a "Seraglio" theme, the Commendatore's introduction in "Don Giovanni," a "Figaro" quartet. Masonic intrigue inspires "The Magic Flute" and the dying Mozart strives to capture his "Requiem." Cardboard figures suggest boxes of the imperial opera and Salieri's rejection of alcohol leads to his addiction to gooey little cakes and creams. Peter Hall's direction abounds with such meaninful details.

Shaffer's perceptions of his case study lead him to myriad issues -- so many that you sense he may have tossed away even more. I found his three hours wholly engrossing but suspect that the second act might be served by tightening. But with richness so rare a quality on our stages, I would regret any loss of this thick, demanding texture in the reworking that is apparently under way.

The figures surrounding Salieri are kept in strict perspective, for we are seeing the whole story through his memory. Beneath the impeccable costumes dare some solid acting skills: Nicholas Kepros as the pale emperor, Patrick Hines as the venal theater director, Paul Harding, Louis Turenne, Gordon Gould and Edward Zang among the uncommonly large cast of 27.

Jane Seymour catches the lusty, loving spirit of Mozart's wife, who would become as solid a keeper of the flame as any of history's fire-fanning widows, one more factual detail Shaffer turns to dramatic value.

Tim Curry, so memorable as Tris tan Tzara of "Travesties" and evidently immortal through "The Rocky Horror Show," is in gloriously alert fettle as the skipping, thoughtless genius.

Finally, it is the bravura performance by Ian McKellen that sparks the whole. Beyond the mercurial details of this infinitely complex performance -- vocally and physically -- McKellen reaches for and embraces the humanity of Salieri. Sly, self-knowing, bitter, mocking, funny and dynamic, his canny poisoner is, above all, human. We see ourselves within him. This is the critical ingredient of McKellen's protean portrait.

Unlike most standing ovations, the response of the house to McKellen's cast bow was not a gradual gathering of standing bodies, but a spontaneous, unanimous rising reaction for a performance you'll never forget.

"Amadeus" is a rich theatrical experience you're not likely to repeat for many a year.