The film is biographical, telling the story of the life and artistic struggles of the French composer Hector Berlioz. Berlioz is shown as a recalcitrant medical student in an anatomy class dreaming of becoming a composer; at a demonstration during a performance at the Paris Opéra conducted by Habeneck; at supper with other young artists (Hugo, Janin, Dumas, Mérimée, Delacroix); and chasing after his future wife Harriet Smithson, after a performance of Hamlet. Also depicted are his life in a garret, while suffering from an illness due to an abscess in the throat; a visit from his mother who curses him; and the composition of the Symphonie fantastique. The film then shows his marital breakdown, the premiere of his opera Benvenuto Cellini, his travels throughout Europe, his second marriage to Marie Recio (called "Marie Martin" in the film), public acceptance in old age and reconciliation with his son.

Jean-Louis Barrault intense et tourmenté, est un inoubliable Hector Berlioz. Une vie de musicien, pleine de luttes d'amours et de drames, de sa jeunesse romantique jusqu'à sa vieillesse honorée... pour imposer son œuvre.

Il giovane Hector Berlioz abbandona gli studi di medicina per dedicarsi alla musica. Si innamora follemente dell'attrice Harriet Smithson, ne fa l'"idea fissa" della sua Sinfonia fantastica e la sposa nonostante l'opposizione della famiglia. La gioia del nuovo amore forse non basta a superare le difficoltà che si incontrano nel mondo musicale; dovrà attendere la vecchiaia per vedere riconosciuta la sua bravura e la sua opera.