What happens to a man whose entire personality undergoes a sudden and profound change? Stephen Marsh is a successful management consultant, happily married and with a daughter at university. Then one day he becomes cold to his wife, is unable to relate to his daughter, is uninterested in his job and his friends. The old Stephen Marsh seems to have been replaced by an intrusive newcomer equipped with new memories and new attitudes. The deuteron-Marsh recalls a previous half-century of life as a writer who shied away from human contact, nurturing a secret which separates him from other people. Incapable of giving love or companionship, the new Marsh struggles to continue his predecessor's lifestyle, torn between a longing for solitude and the bounds of ordinary obligation and affection.