In the preface to the score of his Symphonie Fantastique (1830) Berlioz outlines the story told by the symphony. My additions are in square brackets.
A young musician of extraordinary sensibility and abundant imagination, in the depths of despair because of hopeless love, has poisoned himself with opium. The drug is too feeble to kill him but plunges him into a heavy sleep accompanied by weird visions. His sensations, emotions, and memories, as they pass through his affected mind, are transformed into musical images and ideas. The beloved one herself becomes to him a melody, a recurrent theme [idée fixe] which haunts him continually.
I. Reveries. Passions
First he remembers that weariness of the soul, that indefinable longing, that sombre melancholia and those objectless joys which he experienced before meeting his beloved. Then the volcanic love with which she at once inspired him, his delirious suffering, his return to tenderness, his religious consolations.
II. A Ball
At a ball, in the midst of a noisy, brilliant fête, he finds his beloved again.
III. In the Country
On a summer evening in the country, he hears two herders calling each other with their shepherd melodies. The pastoral duet in such surroundings, the gentle rustle of the trees softly swayed by the wind, some reason for hope which had come to his knowledge recently – all unite fill his heart with a rare tranquility and lend brighter colours to his fancies. But his beloved appears anew, spasms contract his heart, and he is filled with dark premonition. What if she proved faithless? Only one of the shepherds resumes his rustic tune. The sun sets. Far away there is rumbling thunder – solitude – silence.
IV. March to the Scaffold
He dreams he has killed his loved one, that he is condemned to death and led to his execution. A march, now gloomy and ferocious, now solemn and brilliant accompanies the procession. Noisy outbursts are followed without pause by the heavy sound of measured footsteps. Finally, like the last thought of love, the idée fixe appears for a moment, to be cut off by the fall of the axe.
V. Dream of a Witches Sabbath
He sees himself at a Witches Sabbath surrounded by a fearful crowd of spectres, sorcerers, and monsters of every kind, united for his burial. Unearthly sounds, groans, shrieks of laughter, distant cries, to which other seem to respond! The melody of his beloved is heard, but it has lost its character of nobility and reserve. Instead, it is now an ignoble dance tune. Trivial and grotesque. It is she who comes to the Sabbath! A shout of joy greets her arrival. She joins the diabolical orgy. The funeral knell, burlesque of the Dies Irae. Dance of the Witches. The dance and the Dies Irae combined.
(Hector Berlioz, preface to the score of Symphonie Fantastique, 1830.)