The film spans the life of the protagonist, Antonio Salieri, who has desired, since a child, to be a great composer. He makes a "deal" with God if He will grant him this only wish. Salieri is content with his talent and status in court until a musical rival comes to Vienna, Wolfgang Amadeus Mozart. Salieri brims over with jealousy when he hears Mozart's music and realizes God has blessed Mozart with the gift of music, not him. Salieri becomes obsessed with conspiring against Mozart, God's musical "incarnation", and eventually kills him. Only in the end does Salieri realize he has not defeated God, for Mozart's music endures while Salieri's fades into obscurity.
Excerpt of the play from "Amadeus"
by Peter Shaffer (Penguin Books, 1981)