In this tale, a young hydraulic engineer named Victor Hatherley has just gone into business for himself and doesn't as of yet have many clients. His office is visited by a Colonel Lysander Stark who tells Hatherley that he wants him to repair a hydraulic press which is being used to compress "fuller's earth" (clay) into bricks. Stark offers to pay him 50 guineas- an astronomical amount- for the job, but also tells him that it is confidential work which can't be talked about. Hatherley thinks this odd, but can't afford to turn down the job.
The press is located outside of London and Hatherley takes a train out of town and is met by Stark who drives him a considerable distance in a carriage with covered windows. When they arrive at a house, Hatherley is briefly left alone by Stark and a woman appears, warning him that he should get out. Spooked but needing the money, Hatherley stays and Stark returns, taking him to the room where the press is. Hatherley climbs into the press to examine it and finds a metallic deposit on the floor of it which makes it obvious that it's not being used to press fuller's earth. Fed up with all the secrecy, he tells Stark that he'd stand a better chance of repairing the press if he was told the truth about what it was being used for. Instead, Stark locks Hatherley in and turns the press on, intending to crush him to death. Fortunately the mysterious woman reappears and releases him from the press. He is making his way out of the house when Stark realizes that the press was turned off and finds Hatherley, trying to kill him with an axe. Hatherley is forced to jump out a second storey window and while he is clinging to the sill, Stark brings down the axe, chopping off the young engineer's thumb. Hatherley falls, but survives and eventually makes his way back to London.
Hatherley goes to Dr. Watson's office to have his hand treated and bandaged. When he relates what happened, Watson takes him to see Holmes who immediately resolves to find out what is going on.