Dick Young has been lent a house in Cornwall by his friend Magnus Lane, a professor of bio-physics. He has agreed, while he is there, to act as guinea-pig for a new drug that Magnus has discovered; and the small bottles which hold it are waiting for him in the laboratory at Kilmarth.

With no idea of what the result will be, he takes the prescribed dose - and without warning finds himself back in the fourteenth century, in this same Cornish countryside. During the following days he takes successive 'trips'. Each lasts only a short time, but always he is back in the same surroundings. Invisible, inaudible, he finds that he is a kind of alter ago of Roger Kylmerth, steward to Sir Henry Champernoune, lord of the manor of Tywardreath. He witnesses intrigue, adultery, murder, feeling himself personally involved.

Are the experiences hallucinatory, a subconscious escape from dissatisfaction with his own marital life? Or has he really travelled back in time? This is the question Dick asks himself.

More and more he grudges the hours spent away from these people of long ago, resentful of the time that must be given to his loving though suspicious wife and to his stepsons, intruders into his secret life. With immense skill, the tension is kept on both levels, and, as Dick grows ever more obsessed by his trips into the past, so the reader begins to share his addiction, and to find that past and present become inextricably, perilously mixed - until the final, stunning climax.