Russell Yates, the privateer who married Sebastian's former lover Kat Boleyn, is found beside the murdered gem dealer Benjamin Eisler. Yates insists he was framed, and Sebastian agrees to find the killer before the evidence sends him to the gallows.
A missing blue diamond leads through wealthy collectors, occult texts and the disrupted ownership of the French crown jewels during the Revolution. The case forces Sebastian to deal with Kat again and tests the trust he and Hero are still learning to place in one another.