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.