If you've ever seen Peter Shaffer's 1979 play Amadeus or the Oscar-winning 1984 film based on it, you know some of the story of Mozart's Requiem in D minor, K. 626 - but you probably know a clever fiction. Shaffer based his work on an 1830 Pushkin play, Mozart and Salieri, also used as the libretto for Rimsky-Korsakov's opera of the same name. Unfortunately, Pushkin's retelling of history was a web of alternative facts.

It's true, Wolfgang Amadeus Mozart struggled with the composition on his deathbed, completing only two movements and parts of two others before his demise from unknown causes at age 35. It's true Mozart and Antonio Salieri were frenemies. It's even true that considerable skulduggery surrounds the attempted completion of the Requiem, as Mozart's widow, Constanze, tried to have it finished surreptitiously in order to wrest the second half of Mozart's commission from an oddball count, Franz von Walsegg-Stupach. There were rumors Salieri poisoned Mozart, but no physical evidence supports that claim. Salieri denied it fervently before acquiring dementia a year and a half before dying at age 74. Constanze maintained her husband died haunted by the certainty he'd been murdered by the Italian opera establishment, and that he knew he'd effectively written a coda for himself. We don't know for sure whether Mozart wrote all portions attributed to him, as the manuscript is in several hands. Mozart may have dictated to student Franz Süssmayr, or Süssmayr may have written that music himself - at the urging of a desperate Constanze. Sadly, Mozart's Requiem was unprepared for his simple funeral.

What's left after Shaffer's sensational dramatization is the music - the glorious, unforgettable music. Miloš Forman's film of the play vividly explicates the construction of the Sequentia's Confutatis, the lyrics for which plead, "Call me with the blessed. I pray in supplication on my knees, my heart contrite as the dust. Safeguard my fate." To evoke the threat of everlasting hellfire, Mozart composed that eighth movement in A minor rather than D minor. At least 17 different composers have attempted to finish Mozart's Requiem, but thus far his genius shines all the brighter for their efforts.

In 1958, an unknown thief at the Brussels World's Fair managed to tear off a corner of the manuscript. Experts believe the missing corner includes Mozart's final written words (Quam olim d: C," meaning "repeat the ‘Quam Olim' fugue from the beginning"). The piece was never recovered. Such mysteries inform any encounter with the Requiem, so expect a reverent audience for Northwest Sinfionetta's performance of the piece at Tacoma's Broadway Center. The orchestra will be joined by the Seattle Choral Company and preceded by Crane Palimpsest, a 2012 piece composed by Gabriel Kahane. Kahane himself, a talented vocalist, will join the chorus for this performance, under the baton of Eric Jacobsen and vocal direction by SCC's founding conductor and artistic director, Freddie Coleman.

MOZART'S REQUIEM, 7:30 p.m., Saturday, May 20, Rialto Theater, 901 Broadway, Tacoma, $20-$50, 888.356.6040