The finger of Saturn.

Robert Rolt, a landowner who has had a brief career in the Foreign Office but now runs the Dorset estates he has inherited from an older brother. Two years before the book begins his wife Sarah vanished. A man from a secret branch of the Foreign Office comes to him with surveillance film of a woman called Angela Starr who resembles his wife. Rolt is immediately sure it is her. He meets her. She tells him she is an amnesia victim with no memory going back more than a year.