In this story, Mr. Trevor Bennett comes to Sherlock Holmes with a most unusual problem. He is Professor Presbury's personal secretary, and Mr. Bennett is also engaged to the professor's only daughter, Edith.
Professor Presbury is himself engaged to a young lady, Alice Morphy, a colleague's daughter, although he himself is already sixty-one years old. Their impending marriage does not seem to have caused a great scandal; so that is not Mr. Bennett's problem. Nonetheless, the trouble seems to have begun at about the time of Professor Presbury's and Alice's engagement.
First, the professor suddenly left home for a fortnight without telling anyone where he was going. He returned looking rather travel-worn. It was only through a letter from a friend sent to Mr. Bennett that the family learnt that Professor Presbury had been to Prague.
Also, the professor's usually faithful Irish Wolfhound has taken to attacking him on occasion, and has had to be tied up outside. Holmes knows from his study of dogs that this is significant.
Upon returning from Prague, Professor Presbury told Mr. Bennett that certain letters would arrive with a cross under the stamp, and he was not to open these. Until this time, Mr. Bennett had enjoyed the professor's implicit trust and had opened all his letters as part of his job. As the professor said, such letters did arrive, and he gave them straight to the professor. Whether any replies were sent Mr. Bennett does not know, as they never passed through his hands.
The whole household feels that they are living with another man, not the Professor Presbury that they once knew. He has become furtive and sly. There are definite changes in his moods and habits, some quite bizarre; however, his mind does not seem to be adversely affected. His lectures are still brilliant, and he can still function as a professor.
Mr. Bennett observed a curious behaviour in his employer. He opened his bedroom door one night, as he tells Holmes and Watson, and saw the professor crawling along the hall on his hands and feet. When he spoke to Professor Presbury, his master swore at him and scuttled off to the stairway.
Edith Presbury, who arrives at 221B Baker Street halfway through her fiancé's interview with Holmes says that she saw her father at her bedroom window one night at two o'clock in the morning. Her bedroom is on the second floor, and there is no long ladder in the garden. She is sure that she did not imagine this.
The professor brought a small carved wooden box back with him from Prague. One day, as Mr. Bennett was looking for a cannula, he picked the box up, and the professor became very angry with him. Mr. Bennett was quite shaken by the incident.
Mr. Bennett mentions that the dog attacks came on July 2, 11, and 20. Holmes does not mention it aloud at the time, but these are intervals of nine days each time.
Holmes and Watson go to Camford to see the professor the next day. They decide to pretend that they have an appointment, and that if Professor Presbury does not remember making one, he will likely put it down to the dreamworld that he has been living in lately. Things do not go quite this way. The professor is quite sure that he has made no appointment, and confirms this with his embarrassed secretary, Mr. Bennett. Professor Presbury becomes furiously angry at the intrusion, and Watson believes that they might actually have to fight their way out of the house. Mr. Bennett, though, convinces the professor that violence against a man as well known as Sherlock Holmes would surely bring about a scandal. Holmes and Watson leave, and then Holmes confides to Watson that the visit has been worthwhile, as he has learnt much about the professor's mind, namely that it is clear and functional, despite the recent peculiar behaviour.
Mr. Bennett comes out of the house after Holmes and tells him that he has found the address that Professor Presbury has been writing to and receiving the mysterious letters from. The addressee is a man named Dorak, a Central European name. This fits in with the professor's secret journey to Prague. Holmes later finds out from his "general utility man" Mercer that Dorak is indeed a Bohemian, elderly, suave man who keeps a large general store. Before leaving the professor's house, Holmes has a look at Edith's bedroom window, and sees that the only possible way for someone to climb up there is by using the creeper, rather unlikely for a 61-year-old man.
Holmes has formed a theory that every nine days, Professor Presbury takes some kind of drug which causes the odd behaviour. Holmes believed that he became addicted in Prague, and is now supplied by this Dorak in London. Holmes has told Mr. Bennett that he and Watson will be in Camford once again on the next Tuesday. As is usual with Holmes, he does not explain why.
He and Watson show up on the appointed evening, and Holmes suddenly realizes something. He has observed the professor's thick and horny knuckles, and until now, has not made the connection between these, the odd behaviour, the dog's change in attitude towards his master, and the creeper. The professor is behaving like a monkey!
Shortly after the realization, Holmes and Watson are treated to a firsthand display of Professor Presbury's odd behaviour. He comes out of the house, scampers about on all fours, climbs on the creeper, and torments the tied-up dog. Unfortunately, the wolfhound gets loose and attacks the professor. The two of them, with Mr. Bennett's help, manage to get the dog off the professor, but he is wounded badly. Watson and Bennett, who is also a medical man, tend to the professor's injuries.
Holmes then examines the professor's little wooden box after having obtained the key from the now unconscious owner. It contained a drug, as Holmes expected, but there was also a letter there from a man named Lowenstein who, it turns out, is a quack whose help the professor sought out as a way of achieving rejuvenation, which he thought would be advisable if he were going to marry a young woman. The drug is an extract obtained from langurs, and although it has apparently given the professor renewed energy, it has also given him some of the langur's traits, and that is why the dog was attacking its owner.
Good plot, I recommend this book to all readers that appreciate a well written mystery book, mainly featuring Sherlock Holmes. ( )