Veterinarian Erica Merrill becomes involved in a murder investigation when a drug she has prescribed for a dog is used to poison a wealthy widow. Everyone has a motive, money, but only the nine-year-old granddaughter had means and opportunity. Could the child's father have persuaded her to murder her grandmother? It is Erica's knowledge of the effect of the drug on the dog, not the victim, that allows her to solve the mystery.