The storyline is essentially simple. Two unmarried sisters, Lucy and Marcia, live with their father, Dr Knapp, in a house in New York City large enough to accommodate a number of tenants. The doctor is a man of charm, but he's also domineering and selfish, and his daughters are little better than unpaid servants. Lucy doesn't work, and has a history of mental breakdown. Marcia has a job, and serious problem with alcohol.
Dr Knapp also has an eye for the ladies, and his latest affair is with a nurse, Pam Caldwell. Unfortunately for the daughters, he plans to marry Pam in secret, and then turf them out of the house. When they get wind of this, in their desperation they play about with ideas, mostly rather impractical, about murdering either Pam or their father. Then Fate intervenes, and Dr Knapp and Pam are killed in a car crash.
This proves to be anything but a happy ending for the bereaved daughters. Potts is interested in the idea of their moral culpability, the consequences of their "evil wish", even though they have committed no crime. Both Lucy and Marcia display increasingly self-destructive patterns of behaviour and when an unscrupulous photographer called Chuck comes into their lives, disaster beckons...
This is an exceptionally well-written book. The characterisation of the daughters is first-class and the prose sinewy. There aren't any likeable people in the story, but that doesn't really matter. I was gripped from start to finish.