Harriet Smithson is an Irish actress; she is from a theatrical family, yet she spent her childhood resisting their calling. Hector Berlioz is meant to follow in his father's footsteps as a doctor, yet he cannot resist the music within him. When Harriet comes to Paris with an English company to act Shakespeare, Berlioz sees her as Ophelia and instantly adores her. After years of an obsession which produces _Symphonie Fantastique_ (probably Berlioz's best-known work), they eventually meet and begin a powerful, painful love affair.
I have to say first that I did not care for the book's style. Morgan switches between past tense and present tense, often with different styles of punctuation; he interpolates first-person bits in which it seems Harriet is addressing the author directly, and there are even odder bits in which the author is essentially interviewing other composers (Chopin, Mendelssohn) about Hector and Harriet. I've read books where the author's tense changes seemed to mean something (Jo Walton's Lifelode or Rumer Godden's _China Court_, for example), but here, I just found them and the other style variations confusing and self-consciously clever. Every time the style changed, I was bounced out of the narrative and had to work to re-immerse myself.
But dislike of the style aside, I always was able to dive back into the book and keep reading with enjoyment. Morgan does beautifully with the period, with the characters, and most of all, with the portrayal of life as an artist (whether actor or composer). The novel does slow in the last hundred pages or so, because once Harriet and Hector are together, the tension of wondering when that would happen is gone and replaced by a drearier anxiety over how long their relationship will actually last. On balance, though, it's an excellent story, though hampered by an overly elaborate style.