The film is set in Vienna on 9 June 1804, the date of the private, first performance of Beethoven's third symphony, later to be known as the 'Eroica'. The performance, and most of the action in the film, takes place at the palace of one of Beethoven's patrons, Prince Franz Lobkowitz. Midway during the performance, Beethoven tries to get his lover, a widow named Josephine von Deym, to marry him, but she refuses because of the unfair laws regarding child custody – she is a member of the nobility, and cannot marry a commoner without losing custody of her children. Later, composer Joseph Haydn, now old and feeble, arrives just in time to hear the last movement of the symphony.
During the last few minutes of the symphony, the film flashes forward, and we see Beethoven going to dinner with his pupil, Ferdinand Ries, where he is told that Napoleon has just declared himself Emperor of France, thereby completely betraying Beethoven's faith in him. In a rage, he crumples up the title page of his symphony, which he originally intended to call the "Bonaparte". As he leaves the performance, Haydn is asked his opinion of the symphony, which he describes as "quite new", and then utters his now-famous and prophetic comment, "From this day forward, everything [in music] is changed". The film ends on a grim note; as the performance of the Eroica ends, Beethoven looks at his audience and is momentarily unable to hear any natural sounds – an ominous sign of his approaching deafness.