Richard and Natalie are a brother and sister who have been brought up by different parents and have never been close. At the start of a long, hot summer in present-day London, their paths cross once more, only now their roles are curiously reversed. While Richard is full of doubts about his previous way of life and has taken a poorly-paid job, a seemingly self-possessed Natalie is living in Home Counties ease and splendour with her rich and eccentric husband Sinclair. Richard is transfixed by the apparent change in his now beautiful sister and when the two finally meet in private in Richard's flat their mutual attraction runs out of control.