Miss Priscilla Carmody and her niece Connie are dismayed when their overbearing cousin Edris Tidson and his younger wife arrive from the Canary Islands and announce their intention to move in. Going on holiday to the Winchester area where a naiad has been spotted in the River Itchen, a further problem arises when two boys are found murdered in the area where Tidson had been seen prowling. Old friend Mrs. Bradley, who is staying at the same hotel, decides to investigate the matter with her niece Laura and close friend Detective Inspector David Gavin.