In the summer of 1889, Watson receives an unusual patient. A train station worker brings a young man missing his thumb. The injury is recent, and the patient is distraught. After treating him, Watson takes the man to meet Holmes.

The patient, Victor Hatherley, is an orphan, living alone. By profession, he is a hydraulic engineer and is in the process of establishing a private practice. His first client in a long time is a strange man who needs a consultation on a hydraulic machine. The offered payment is extremely generous, but the conditions of the job are extremely peculiar. The young man is required to come at night to a house in the countryside in complete secrecy. The job is suspicious, but the pay is too good to pass, so the young man agrees and does as he is told.

Once at the house, he is taken to a huge press, the size of a room, which is malfunctioning. He is able to quickly discover the problem but realizes that the setup is different from what his client had told him about the machine’s purpose. At that point, his client attempts to kill him by locking him inside the press chamber and activating the machine, but a mysterious German woman, residing at the house, helps him escape through the window.