An old man named Antonio Salieri ends up in an asylum after cutting his throat and screaming that he killed Mozart. In the asylum, Father Vogler, a young priest, visits Salieri and urges him to confess the thoughts that are tormenting him. In response to the priest's pleas, Salieri begins a long narrative that begins in his childhood and ends with Mozart's death.
As a teenager, Salieri is passionate about music, but his domineering father makes it impossible for him to pursue his dreams. His father dislikes music and mocks not only Salieri's passion for music, but also Salieri's musical icon, the six years old prodigy, Mozart. Feeling helpless, young Salieri turns to God. He promises chastity and devotion if God turns him into an exceptional musician whose name and work transcends time. Soon after, Salieri's father dies, and Salieri believes that God has accepted his promise. The death allows Salieri to study music in Vienna and within a couple of years Salieri becomes a revered composer in Emperor Joseph II's court. Throughout this time in his life, Salieri keeps his promise to God. He is religious, chaste, and generous to others.
Salieri's happiness in Vienna comes to an end when Mozart comes to the city. Salieri encounters Mozart at a party that takes place at the residence of Prince-Archbishop Colloredo of Salzburg. Mozart acts inappropriately with a young woman, and this shatters the grand image that Salieri has of him. Still, Salieri approves of Emperor Joseph's decision to hire Mozart to compose a German opera for the national theater. Salieri soon regrets this decision when Mozart insults a piece that Salieri writes to welcome him to the emperor's palace. Mozart insults Salieri further by sleeping with Katherina Cavalieri, the woman whom Salieri admires.
Mozart's presence in Vienna puts a strain on Salieri's relationship with God. Salieri becomes conscious of his mediocrity because of Mozart, and this sense of mediocrity makes Salieri feel betrayed by God. Salieri realizes that, despite his esteemed position in the emperor's court, God has not, after all, fulfilled the prayers from his teenage years. He realizes bitterly that Mozart, the vulgar and infantile man, is the exceptional talent, not him. These revelations cause Salieri great pain, and they lead him down a dark path.
As Salieri descends into darkness, he takes advantage of Mozart's precarious living situation in Vienna. After Mozart severs ties with Prince-Archbishop Colloredo, Mozart only has tutoring positions and his performances as sources of income to support his extravagant lifestyle. Salieri knows this and spreads a rumor that Mozart has inappropriate relationships with pupils. The rumor costs Mozart not only the opportunity to tutor Princess Elizabeth, the emperor's niece, but also the opportunity to tutor other young women in Vienna. Salieri also takes advantage of Mozart's naive nature. Mozart does not realize throughout the film that a large portion of his problems stem from Salieri. Instead, he begins to view Salieri as a confidant.
Leopold, Mozart's father, comes to Vienna, but he is not able to do much for his son. He had tried to salvage Mozart's relationship with the prince-archbishop, but Mozart's rash decisions to stay in Vienna and to marry Constanze, the woman who was his companion at the party held at the residence of the prince-archbishop, cause Leopold's efforts to result in failure. Furthermore, Leopold does not stay long in Vienna. He leaves after arguing with Constanze over Lorl, a young maid who claims to have been paid by a mysterious figure to provide services to Mozart's household. Constanze readily welcomes Lorl to Mozart's household, but Leopold is suspicious of Lorl, and his instincts turn out to be right. Lorl is Salieri's spy.
Lorl helps Salieri sabotage Mozart's work. She informs Salieri when Mozart and Constanze are not at home, and Salieri uses the information to break into Mozart's residence. At Mozart's residence, Salieri learns that Mozart is working on an opera based on Pierre Augustin Beaumarchais' play, The Marriage of Figaro. Salieri brings this information to Count Orsini-Rosenberg and Kapellmeister Bonno, who were, from the start, against Mozart being welcomed to the emperor's court. Count Orsini-Rosenberg and Kapellmeister Bonno then bring the information to the emperor. Mozart manages to escape censorship through persuasion and luck. After censorship fails, Salieri employs other methods to hurt Mozart's work. Using his influence in both Vienna society and in the emperor's court, Salieri makes sure that Mozart's next opera, Don Giovanni, only receives five performances at the national theater. Mozart's financial issues begin to take a mental and physical toll on him.
As the film continues, Salieri takes his need for revenge further. After seeing Don Giovanni, which highlights Mozart's guilt and grief towards a recently deceased Leopold, Salieri deduces a way to end Mozart once and for all. Wearing a mask and black cape that Leopold once wore to a party, Salieri approaches Mozart and commissions him to write a Requiem. Ironically, Salieri plans on taking credit for the Requiem when he performs it at Mozart's funeral after he kills Mozart. Mozart is terrified by the masked figure, but he accepts the work since he is in desperate need of money. Emanuel Schikaneder, an actor, also commissions Mozart for work.
The pressure that Mozart feels from both of his employers worsens his already weak state. Mozart ends up completing The Magic Flute, the opera he promised Schikaneder, but he dies before finishing the Requiem that he promised the masked figure, who Mozart never discovers is Salieri. The only silver lining in Mozart's death is that he is not physically alone when he dies. Constanze, who had left Mozart in the midst of his mental and physical decay, returns with their son, Karl, a few minutes before Mozart departs life. Salieri is also present when Mozart dies.
On the night before Mozart dies, Mozart faints in the middle of conducting The Magic Flute. Salieri, who is present when Mozart faints, takes Mozart home to rest. When Mozart gets home, however, Salieri does not allow him to rest. Salieri entices an exhausted Mozart to work on the Requiem by letting Mozart believe that a knock at the door is the masked figure and by also telling Mozart that the masked figure promises one hundred ducats if Mozart finishes the Requiem by tomorrow night. Salieri even volunteers to help Mozart accomplish the feat. Mozart accepts the help, and he and Salieri work through the night.
Mozart's funeral is not a grand affair. Only his family and a few colleagues attend it. Salieri is one of these colleagues. Mozart does not even get a tombstone. His body is dumped in an unmarked mass grave. Mozart's death leaves Salieri unsatisfied. The Requiem is not only unfinished at the time of Mozart's death, but Constanze locks the unfinished product away. Furthermore, even if Salieri was able to perform the Requiem, Mozart's funeral does not have the type of guests that could elevate a man's status.
Old Salieri reaches the end of his narrative, and Father Vogler has a horrified look on his face. An asylum attendant soon enters the room and takes Salieri out of the room to feed and bathe him. As Salieri is wheeled away, he tells the priest that he is the voice for all mediocrities, including the priest. In the film's last scene, old Salieri absolves the inmates that he sees as the attendant pushes him through a hallway of their mediocrities. The film's last sound is Mozart's laugh.