John and Susan Pearson have rented their usual holiday cottage in France; Susan is particularly looking forward to this holiday as she is recovering from a nervous breakdown. Shortly after their arrival, the cottage's owner turns up to check that everything is in order. She runs out of cigarettes and John leaves to buy her some. Thus begins Susan's confusion and terror, starting with the appearance of a man who says he is John and culminating in her being taken to the mental wing of the local hospital. It transpires that Susan is about to inherit all her late father's money, on condition that she is "of sound mind'. Is she the victim of an elaborate plot?