Regency England, August 1812. Sebastian's plans to escape the heat of London for a honeymoon are shattered when the murdered body of Hero's good friend, Gabrielle Tennyson, is discovered drifting in a battered boat at the site of a long-vanished castle known as Camlet Moat. A beautiful young antiquarian, Miss Tennyson had recently provoked an uproar with her controversial identification of the island as the location of Camelot. Missing and presumed also dead are Gabrielle's two young cousins, nine-year-old George and three-year-old Alfred.

Still struggling to define the nature of their new marriage, Sebastian and Hero find themselves occasionally working at cross-purposes as their investigation leads from London's medieval Inns of Court to its seedy back alleys, and from grand country homes to rural enclaves where ancient Celtic beliefs still hold sway. As he probes deeper, Sebastian also discovers dark secrets at the heart of the Tennyson family, and an enigmatic young French lieutenant with a dangerous, mysterious secret of his own.

Racing to unmask a ruthless killer and unravel the puzzle of the missing children, Sebastian and Hero soon find both their lives and their growing love for each other at risk as their investigation leads to Hero's father, who is also Sebastian's long-time nemesis... and to a tall, dark stranger who may hold the key to Sebastian's own parentage.