England 1912. Danby Hall is the only home Lady Cecilia Bates has ever known. Despite the rigid rules of etiquette and her mother the Countess of Avebury's fervent desire to see her married off, Lady Cecilia can't imagine life anywhere else. But now, with an agricultural depression sweeping the countryside, the Bates family's possession of the hall is suddenly in peril.

A possible solution arrives in the form of the imperious American heiress Annabel Clarke. The Earl and Countess of Avebury are determined that Cecilia's brother, Patrick, will win Annabel's hand in marriage--and her fortune along with it. To help the lackluster Patrick in this pursuit, the Bates and their staff arrange a grand house party upon the heiress's arrival.

When a guest dies after sipping from a glass meant for Annabel, it's clear the Bates have a more poisonous problem on their hands than a lack of chemistry. As the scandal seizes Danby, Cecilia sets out to find the culprit, with help from Annabel's maid, Jane, and Jane's curiously intelligent cat, Jack.

After the poison that someone had stashed away inside the manor is discovered, Cecilia is left with two possibilities: Either a resident of Danby snapped and tried to kill the arrogant heiress, or the threat is coming from one of their guests, who would love to see the Bates family's decline become permanent.