Rosemary and Laura are employed to help re-create an original Victorian planting in an historic Avenue garden. There they meet a variety of characters for whom the central London park plays a regular part in their daily life. The beauty and peace of the Avenue is shattered when Harley Street psychiatrist, Pavel Volkar, is found murdered. Rosemary and Laura adopt Mr Volkar's bereaved three-legged dog Holly, who seems to have more than a canine understanding of events. The mystery surrounding the accident in which Holly lost her leg appears to be key to solving the murder mystery.