Even if the world is falling to pieces, when the treasure of the heart is discovered, a fight is needed to keep it.

King's agent, Bentley Courtenay—Lord Castlereagh—has been sent on his first solo mission to rescue a widow, who's been abandoned in the dense jungles of Venezuela. He intends to bring her back to England to answer questions regarding the poisoning death of a peer. Yet what awaits him in the on the Orinoco River has him wondering why the devil he ever signed up to work for the Crown.

Widowed Cora, Lady Trammel, is on a quest to locate a deadly orchid and destroy every last bloom before the toxin she accidentally created can fall into the wrong hands. When she encounters a king's agent who fires her imagination, she declines his demand to return to London. The fate of the world is at stake, and she has to fix her mistake, for her discovery has the capacity to kill every human on Earth.

In the course of the journey, desire flares between Bentley and the older, more experienced Cora, but she's not looking for love. While he loses his heart, evidence of the fabled city of El Dorado comes to light, and their mission takes a critical turn into dangerous adventure, so matters of love are shelved. Pursued by threats, time is running out to destroy the deadly toxin and protect the world's greatest treasure from men who mean both to use both for their own insidious gain.