The life story of Wolfgang Amadeus Mozart as told by Antonio Salieri. To be a bit more specific, Salieri always envied Mozart his musical talent and wished he could be like him. After Toni's father passes on, Toni moves to Vienna and becomes so prolific a musician that he is elevated to the rank of court composer. The Austrian Emperor hears of Mozart and decides to commission an opera from him. So Wolfy moves to Vienna, his opera (Abduction from the Seraglio, I believe) is so astounding that Toni's jealousy begins to grow. Mozart stays in Vienna, prolonging Salieri's agony, writing music so perfect that Salieri likens it to the very voice of God.
Eventually though, too many parties, too much hard work and not enough rest affects Wolfy's health and Salieri decides to make the most of his enemy's precarious condition by working him to death. Literally. Disguised as Mozart's father, Salieri commissions a requiem mass with the proviso that the sooner he finishes it, the more money he will get. Salieri plants a spy in Mozart's home in the form of a maid who reports back that Mozart is working like a madman, with his deteriorating health nudging him closer to the grave every day. Good news as far as Salieri is concerned. However, the piece Mozart is spending his time on is an Opera (Die Zauberflöte).