A man named Trevor Bennett, the personal secretary of one Professor Presbury, comes to Holmes with a most unusual problem. He is engaged to the professor's daughter, Edith, and the Professor is himself engaged to a young lady, Alice Morphy, although he himself is already 61 years of age. Their impending marriage has caused no scandal, but the trouble seems to have begun at about the time of the engagement. First, the professor suddenly left home for a fortnight without telling anyone of his destination, with the family later learning that he had been to Prague. Upon returning, Presbury unprecedentedly forbid Mr Bennett from opening certain stamped letters. The professor had also brought a carved wooden box from Prague, and became very angry with Bennett for touching it.

The whole household also observes severe changes in the man's personality: he has become furtive and sly, with changes in his moods and habits, some of which are quite bizarre, though not impeding his functions. Bennett notes particularly strange incidents: one, in which he saw the professor crawling along the hall on his hands and feet and his master swore him off; and another, witnessed by Edith Presbury, who saw her father at two o'clock in the morning at her bedroom window on the second floor.