It is 1908, and a weary Asher, traveling home from his tiresome duties as executor of a relative's estate, spots Charles Farren, vampire Earl of Ernchester, clearly involved in some intrigue with mercenary and enemy spy Ignace Karolyi. Asher, who had left the secret service years ago to wed Lydia, reluctantly trails the pair to Paris, wiring Lydia to alert her to the danger.
But Lydia is aware of something Asher doesn't know: an obscure footnote in one of her medical journals clues her into the fact that the safe house Asher is planning to use is in the hands of a double agent, actually in league with Karolyi. To save her husband, Lydia seeks the assistance of the oldest of the London vampires: the enigmatic and haughty Simon Ysidro.
Ysidro agrees to help, to keep Farren from forming an alliance with humans, but on one condition: over Lydia's strenuous objections, he recruits a drab but romantic-minded governess, Margaret Potton, to travel with them as Lydia's chaperon. The three of them trail Asher across Europe to Constantinople, as he joins forces with Farren's strong-willed and alluring vampire-wife, Anthea.
In Constantinople, the trail dies, as Asher has been kidnapped by the vampire lord of the city, who is struggling to keep control as a rival seeks to destroy him. Asher tries to uncover his own role in the power struggle, for this may hold the key to his survival.
Meanwhile, Lydia and Ysidro explore the city, Lydia through Asher's diplomatic connections, Ysidro through his nocturnal prowlings, seeking any clue as to where Asher and Charles have disappeared to, even as Margaret's obsession with Ysidro and jealousy of Lydia endanger them all.
Only by unwinding the threads of money trails, strange business transactions, and a series of increasingly gruesome murders can Ysidro and Lydia find Charles and Asher, and escape the city alive.