Violet Smith has recently started a new job as music teacher to Robert Carruthers, staying during the week at his house in the country. She consults Holmes because she is worried about being followed once a week on her way to the railway station by a man on a bicycle. Carruthers asks Miss Smith to marry him, but Holmes sees a sinister connection between him and the stalker.

An unhappy wife learns that her husband, Colonel Barclay, cheated her earlier fiance, leaving him disabled and in exile. After a fierce argument between them, the Colonel's dead body is found next to his unconscious wife in a locked room with no key, and with curious animal tracks outside the window. Nancy Barclay remains in a coma.Holmes investigates and finds a story of betrayal and a mystery worthy of his powers.