In 1920s London, the Honourable Cressida Fawcett, known for her love of interior design and martinis, visits a friend's mansion in the countryside, recently the site of a diamond heist. After the death of a chandelier cleaner named Harry, which initially appears to be an accident, Cressida suspects foul play. Discovering a torn piece of silk near the body, she links the murder to the theft, believing the diamonds were hidden in the chandelier. As the investigation unfolds, another guest is murdered, trapping everyone in the mansion with a killer among them. Cressida will have to use her keen observation skills to solve the case before another life is lost.

Cressida Fawcett attends a fancy-dress ball at Penbeagle House on the Cornish coast, anticipating a relaxed time with friends and celebrating Lord Canterbury's engagement. However, the event turns tragic when Lord Canterbury dies of poisoning in front of the guests. Teaming up with Detective Chief Inspector Andrews and her pug Ruby, Cressida investigates the murder, uncovering motives among the guests and staff. Following another death, Cressida races to find the killer, using clues hidden in unexpected places to solve the mystery.

Cressida Fawcett's stay at Ayrton Castle in the Scottish Highlands is interrupted by the murder of Hamish Glenkirk, the castle's former owner, found dead in a locked turret room. With Detective Andrews delayed, Cressida takes on the case, scrutinizing the hunting party for suspects. The mystery deepens with another murder, prompting Cressida to navigate through castle secrets and possible motives, using her sharp wit to identify the killer before another victim falls.

After a trip to the Scottish Highlands, Cressida Fawcett's journey back to London on an express train is disrupted by the murder of industrial tycoon Lewis Warriner. Working with Detective Andrews, they interrogate fellow passengers to uncover the murderer. As suspicions mount and another gunshot is heard, Cressida faces the challenge of solving the case with Andrews incapacitated, striving to prevent further deaths before the train reaches its destination.

Visiting her aunt in Winchester, Cressida Fawcett discovers the murdered body of Anthony, the verger, in the cathedral's crypt, with her aunt's maid, Nancy, standing over him. Despite Nancy's plea of innocence and Detective Andrews' quick judgment, Cressida believes there's more to the story. Investigating Anthony's enemies and uncovering cathedral secrets, she finds a hidden passageway, suggesting a different escape route for the murderer. Cressida races against time to prove Nancy's innocence and solve the murder, facing skepticism from the police.