Sebastian and Hero's honeymoon is interrupted when Hero's friend Gabrielle Tennyson is found murdered in a boat at Camlet Moat. Gabrielle's two young cousins have disappeared, and the household's conflicting accounts make it unclear whether the children were taken, killed or hidden.
The couple investigate Arthurian legends, family inheritance and the activities of an enigmatic French officer. Their search for the missing children also brings Sebastian closer to a stranger who may know the truth about his parentage.