Watson has only brought Holmes two cases in his life and this is one of them, a particularly weird problem that emerges during the summer of 1889.
This would be soon after Watson's marriage, when he's working on expanding his medical practice.
One morning at around 7am, two men come to his house/office from nearby Paddington Train Station.
Watson rushes down to help. A guard, an old friend of Watson's, says he's brought him a new patient and then takes off without waiting for thanks.
The new patient is a young guy, around 25, dressed in tweed and holding a bloody handkerchief over his hand.
He apologizes for coming so early, but he has just arrived by train after a serious accident the night before. His name is Victor Hatherley, and he's a hydraulic engineer – someone who works on designs related to fluid mechanics, such as pumps, turbines, canals, dams, and bridges.
After a brief, random fit of hysterical laughter and a glass of brandy, Hatherley calms down and admits that he's had a horrible shock.
Hatherley takes the handkerchief away from his hand and shows Watson that his thumb has basically been torn off. Watson guesses that it was the work of a heavy, sharp instrument and Hatherley agrees that it was a thing like a cleaver.
Watson dresses the wound and listens as Hatherley tells him that he plans to go to the police over his bizarre experiences.
Watson advises that Hatherley see his friend, one Mr. Sherlock Holmes, about his case. Hatherley is psyched, having heard of Holmes's reputation as an all-around genius, and they travel together to Baker Street.
Holmes is in the middle of breakfast when Watson and Hatherley arrive. Holmes invites Hatherley to lie down and rest while he tells his story.
Hatherley starts out: he's a bachelor with no family and he lives in London. He apprenticed for seven years as an engineer and is now running his own practice. Unfortunately, his business has been pretty terrible: he has only earned 27 pounds (around $3,500 today) over the past two years.
So you can imagine that Hatherley is pretty thrilled when, the day before, a middle-aged man with a slight German accent named Colonel Lysander Stark walks into his office with a job offer. Stark does say something a little ominous, though: he says that Hatherley has been recommended as a man who can keep a secret.
Stark makes Hatherley swear secrecy several times before continuing. He also appears to know that Hatherley is unmarried and has no living parents.
Hatherley is disgusted by Stark's behavior and manner, but – he really needs cash, so...
Stark offers Hatherley 50 guineas (around U.S. $6,820 today) – in other words, double what Hatherley has made in two years of business – for one night's work. Yippee!
Stark has a hydraulic press that's not working properly, and he wants Hatherley to look at it. He asks Hatherley to come to the countryside that evening, on the 11:15 train. Hatherley will have to spend the night at Stark's place to catch the morning train.
Hatherley finds this whole late night thing a little weird, but the money is too tempting, and he agrees.
The explanation Stark provides for his secrecy is this: he has found a deposit of valuable fuller's earth, a kind of clay used in filtering vegetable and animal oils, on his property.
The two estates next to his have even bigger fuller's earth deposits, which his neighbors don't know about.
Stark wants to buy their property before they find out there's valuable clay on their land but, to do so, he has to earn some more money.
Stark has decided to do this by quietly processing his own fuller's earth deposit, using a press to shape it into blocks.
He doesn't want any of his neighbors seeing Hatherley because then they'll think, oh, hydraulics engineer = press = something valuable on Stark's land = perhaps there's something valuable on their own land too? In other words, he wants to keep his press a secret.
Hatherley accepts this information, except for one thing: he doesn't get why you would use a press on fuller's earth, which is mined like gravel from a pit.
Stark pooh-poohs this: he says "we have our own process" (Thumb.85) and more or less leaves it at that.
Hatherley agrees to come down, and Stark heads out.
That evening, Hatherley arrives at the Eyford station. He's the only passenger to get off the train, and is met by Stark himself, who drags him off into a carriage.
They drive for at least an hour in the sealed carriage, and Hatherley can't see either the countryside they're driving through or, when they arrive, much of Stark's house.
The house shutters are also barred, so Hatherley can't see the house's surroundings when he's inside.
Hatherley starts to get a little uneasy: he has no idea where he is, and the house itself is creepy. It's utterly silent except for the sound of a ticking clock.
Suddenly, out of nowhere, the door opens and a woman comes in. She looks frightened, and says (in accented English), "I would go. I would go. I should not stay here. There is no good for you to do" (Thumb.102).
Hatherley is intrigued by this warning, but he's also stubborn. He really wants that money, and he's gone to all the trouble of going out to this place in the middle of nowhere anyway, so Hatherley doesn't want to leave right away. The woman hears a door slamming and, seeing that Hatherley won't leave, vanishes out the door.
Stark comes in with a short, fat guy named Mr. Ferguson. They offer to bring Hatherley to the press, which is in the house.
Hatherley is surprised that the press isn't outside, but Stark basically tells him to shut up: all they want is for Hatherley to look at the press and see what's wrong with it.
They go upstairs and Stark unlocks a door to a tiny room that the three of them can hardly fit into at one time.
Stark tells Hatherley that they are now, in fact, inside the press. If it were turned on, the ceiling would come down with crushing force.
The problem with the press is that it's not producing as much pressure as it should be. Hatherley examines the whole thing and discovers a leak in one of the cylinders of the mechanism.
Hatherley points out what's wrong to Stark & co. and tells them how to fix it.
Hatherley is also now absolutely sure that this whole fuller's earth thing was a big lie; the press shows signs of metal crust, which he finds pretty suspicious.
Stark finds Hatherley examining the press's residues and realizes that Hatherley knows Stark is lying (phew, this is getting complicated!).
Stark is not happy, and, in a classic evil villain gesture, can't leave the room without a little jab at Hatherley: "Very well [...] you shall know all about the machine" (Thumb.121). Insert evil cackle here.
Stark slams the door to the press, locks it, and starts the mechanism.
Luckily for Hatherley, while the floor and ceiling of the press are made of metal, the walls are wooden, and there's a hidden panel through which he manages to escape.
Waiting for him on the other side is the German woman who warned him before. She leads him to a window that he can escape through.
Stark has, meanwhile, managed to arm himself with a big cleaver.
Stark chases after Hatherley and almost catches him, but the German lady (who, we find out, is called Elise) holds him back.
Stark breaks free and hacks at Hatherley's hands where he's gripping the windowsill.
Hatherley falls into the garden below and staggers away. He faints (shock? blood loss?) after discovering that his thumb's gone and wakes the next morning to find himself near the local train station – he has vague memories of being carried there, but no idea by whom.
Hatherley hightails it back to London, where he asks for a doctor, meets Watson, and, well, here we are.
Holmes, having heard all this, pulls out a newspaper ad for a missing person he had clipped, searching for another young hydraulics engineer who has vanished from the area. Holmes connects the disappearance of this guy Jeremiah Hayling with Stark, pointing out that he was probably the previous person Stark contacted about his press.
Holmes tells Hatherley they have to go to Scotland Yard immediately before continuing their investigation.
After visiting Scotland Yard, where they collect Inspector Bradstreet and a plain-clothes officer, Watson, Holmes, and Hatherley head to Eyford station.
They spend the trip trying to imagine which direction the house could have been, given the twelve-mile drive Hatherley reported. All of them have different guesses.
Holmes says that the house is probably right next to the station, and the drive was six miles away and then six miles back to confuse Hatherley's sense of distance.
Holmes adds that the gang is counterfeiting silver with the press, and Bradstreet suggests that it's the same gang Scotland Yard has been tracing but never caught.
And they're not caught by the end of the story, either – the group arrives at the station to find a house on fire, one that Hatherley immediately recognizes.
Apparently, the oil lamp Hatherley had been carrying when Stark tried to press him to death set the wooden walls on fire.
But it would appear that the gang managed to escape with their lives: apparently, they fled in a cart filed with bulky boxes, never to be seen again.
The firemen did discover a freshly-cut human thumb on a windowsill on the second floor, plus lots of nickel and tin in the outhouse. This proves both Hatherley's story and Holmes's idea about the silver counterfeiting.
Footprints in the mud around the house suggest that it was the woman, Elise, and the silent English "manager" of Stark's who carried Hatherley to the station. Holmes assumes that the manager is probably not as cutthroat and cruel as Stark, and doesn't want to see Hatherley murdered.
Hatherley remarks, as they set off for London, that he's lost both a fifty-guinea fee and a thumb, and what has he gained?
Holmes reassures him that at least Hatherley has a story he can dine out on for the rest of his life.