Robert Cavendish was the former owner of a quarry - the same quarry where Matthew Draper and Emily Beavis were killed. After Matthew Draper was killed at the quarry, his wife and daughter wanted to get revenge on Robert and the rest of the Cavendish's for the death of Matthew.
Robert had ordered the dynamite to be checked after it didn't go off. Ian Frasier sent Matthew to investigate the dynamite which suddenly exploded, killing him. After this event, Matthew's daughter Patricia and his wife (Mrs. Wilson) set out to destroy Robert for taking Matthew away from them.
The first step of their plan was to kill Emily Beavis so that Mrs. Wilson could get closer to Robert. After Mrs. Wilson killed Emily by pushing her over the edge of the quarry, she then moved on to the next target which was Robert's wife Tara. After Mrs. Wilson killed her with a bat, they then moved on to killing Robert. Mrs. Wilson set the scene by serving him wine laced with Valium. Patricia arrived with a knife and slit one of Robert's wrists to make it look like suicide, but before she could finish, DCI Barnaby and DS Troy arrived and Robert forced Patricia off him, accidentally stabbing and killing her in the process. Mrs. Wilson was then arrested.