Berlioz wrote semi-autobiographical programme notes for the piece that allude to the romantic sufferings of a gifted artist who has poisoned himself with opium because of his unrequited love for a beautiful and fascinating woman (in real life, the Shakespearean actress Harriet Smithson, who in 1833 became the composer's wife).

The artist's reveries take him to a ball and to a pastoral scene in a field, which is interrupted by a hallucinatory march to the scaffold, leading to a grotesque satanic dance (Witches' Sabbath).

The Symphonie fantastique is a piece of programme music that tells the story of a gifted artist who, in the depths of hopelessness and despair because of his unrequited love for a woman, has poisoned himself with opium.

The piece tells the story of the artist's drug-fuelled hallucinations, beginning with a ball and a scene in a field and ending with a march to the scaffold and a satanic dream.

The artist's passion is represented by an elusive theme which Berlioz called the idée fixe, a contemporary medical term also found in literary works of the period.

If the symphony is performed in isolation in a concert, this arrangement is no longer necessary; it is even possible to dispense with distributing the programme, retaining only the title of the five movements.

"[7] Attending a performance of Shakespeare's Hamlet on 11 September 1827, Berlioz fell in love with the Irish actress Harriet Smithson, who played the role of Ophelia.

She did not attend the premiere, given at the Paris Conservatoire on 5 December 1830, but she heard Berlioz's revised version of the work in 1832 at a concert that also included its sequel, Lélio, which incorporates the same idée fixe and some spoken commentary.

By a singular oddity, the cherished image never presents itself to the artist's mind except in connection with a musical idea, in which he finds a certain passionate, but noble and timid character like that which he attributes to the beloved object.

A long, slow introduction leads to an Allegro in which Berlioz introduces the idée fixe as the main theme of a sonata form comprising a short exposition followed by alternating sections of development and recapitulation.

It begins with a mysterious introduction that creates an atmosphere of impending excitement, followed by a passage dominated by two harps; then the flowing waltz theme appears, derived from the idée fixe at first,[20] then transforming it.

This pastoral duet, the scenery, the slight rustling of the trees gently stirred by the wind, some hopes that he has lately found reason to conceive, all conspire to restore to his heart an unaccustomed calm, to give to his ideas a more cheerful colour.

[24]Berlioz claimed to have written the fourth movement in a single night, reconstructing music from an unfinished project, the opera Les francs-juges.

Before the musical depiction of his execution, there is a brief, nostalgic recollection of the idée fixe in a solo clarinet part, as though representing the last conscious thought of the soon-to-be-executed man.

The beloved melody reappears again, but it has lost its character of nobility and timidity; it is no more than a dance tune – ignoble, trivial and grotesque; it is she who is coming to the sabbath ... Roar of joy as she arrives ... She joins in the diabolical orgy.

François-Joseph Fétis, founder of the influential Revue musicale wrote of it approvingly,[28] and Robert Schumann published an extensive, and broadly supportive analysis of the piece in the Neue Zeitschrift für Musik in 1835.

[27] He had reservations about "wild and bizarre" elements and some of the harmonies,[29] but concluded: "in spite of an apparent formlessness, there is an inherent correct symmetrical order corresponding to the great dimensions of the work – and this besides the inner connection of thought".

[31] By the middle of the 20th century the authors of The Record Guide, calling the work "one of the most remarkable outbursts of genius in the history of music", commented that it was a favourite with the public and with great conductors.

Sir Thomas Beecham, a lifelong proponent of Berlioz's music, remarked on the originality of the work, which "broke upon the world like some unaccountable effort of spontaneous generation which had dispensed with the machinery of normal parentage".