Roger and Alison are stepbrother and sister. Alison's father died and her mother Margaret has married Clive, a businessman and former RAF officer. Clive's former wife was notoriously unfaithful, bringing shame to the family and a particular kind of pain to his son, Roger. To bond the new family together they are spending a few weeks of the summer in an isolated valley in Wales, a few hours' drive from Aberystwyth. They occupy a fine house formerly owned by Alison's father, subsequently transferred to her to avoid death duty. He in turn inherited it from a cousin, Bertram, who died there in mysterious circumstances around the time Alison was born.