Narcissus in Chains, Cerulean Sins and Incubus Dreams share one common thread….sex. While sex has always been a prevailing theme throughout the series (who is Anita sleeping with now?), it becomes a part of the plot device when Jean-Claude’s incubus origins are transferred to Anita through their marks.
While I could go into the plot, and give a summary of what the bad guys want and what the action entails, I won’t because the plot only touches on the underlying theme of sex that Hamilton begins to include in her books. Plus, you should read it. If I give you the main plot devices and tell you what goes on, where will you get the motivation to read it?