There exist multiple versions of the play Amadeus, which was revised on numerous occasions by the playwright. Furthermore, the 1984 film adaptation also differs significantly from the original play. The following is an attempt to summarise the fundamental shared details of the various versions of the narrative as closely as possible.
It is the early nineteenth century. Antonio Salieri, a once-respected composer, is now a bitter old man whose name and works have been mostly forgotten. In a last-ditch effort to achieve some recognition, he claims that he is the murderer of Wolfgang Amadeus Mozart, the beloved musical prodigy who died as a young man and whose works continued to be played and admired. To convince his skeptical audience, he proceeds to tell them the story of his dealings with Mozart as a younger man, and how his machinations led the other man to ruin.
As a young man, Salieri prays to God and makes a deal that in exchange for success as a composer, he will devote his life to God's service. Salieri does achieve the success and renown he desires, eventually rising to the position of court composer to Emperor Joseph II of Austria, but while his work becomes famous it tends to only receive modest and polite acclaim, and he is frustrated that despite his efforts he never manages to create a truly great piece. However, his life changes on the day when he meets Mozart, who even as a young man has produced revolutionary music. An admirer of his work, Salieri is disappointed to discover that Mozart in person is boorish, uncouth and immature, but reacts in disbelief when he is given proof of Mozart's almost impossible musical genius — the younger man can compose impossibly beautiful music seemingly instantaneously. Salieri cannot reconcile Mozart's glaring personal flaws with the almost divine nature of his abilities, and can only conclude that God is mocking him by giving such a man transcendent abilities while giving Salieri himself, despite his lifetime of faithful devotion, only enough skill to allow him to recognise Mozart's genius — and how dwarfed in comparison he is by it.
Furious and vengeful, Salieri breaks his pact with God and spitefully vows to destroy Mozart, who he views as God's chosen vessel. He proceeds to befriend the younger man and represent himself as a mentor figure, while secretly doing everything he can to smear Mozart and undermine his standing at court and within their profession. Mozart's fortunes are not helped by his own feckless nature, which lead to increasing dependency on alcohol, his turbulent and passionate personal life with his beautiful wife Constance, and the revolutionary nature of his music, which frequently baffles those around him. Mozart's works are frequently rejected by the court and the public, partly due to being ahead of their time and partly through the manipulations and machinations of Salieri, leading him to increasingly desperate, elaborate and incredible attempts at earning their favour; this results in the creation of such masterpieces as The Marriage of Figaro and The Magic Flute. Salieri alone, however, continues to recognise Mozart's genius and the beauty of his music, which only fuels his obsession with destroying Mozart further.
Salieri's manipulations and sabotage eventually leads Mozart, who finally pushes himself too hard, to suffer a severe physical and emotional collapse as his marriage to Constance falls apart. His health destroyed, Mozart dies, but not before completing his final Requiem (in some versions, Salieri aids Mozart in its composition, intending to steal the work and pass it off as his own). This brings Salieri's plot to fruition, but any satisfaction Salieri achieves on the success of his efforts is short-lived — in death, Mozart's reputation only grows as the world finally comes to recognise his true genius, and his music lives on long after he has passed away. Salieri may have destroyed the man, but the true divinity was in the music; furthermore, his claims that he is Mozart's murderer are rejected. Defeated, Salieri returns to obscurity, doomed to live the remainder of his life in mediocrity and failure.
