Sherlock Holmes is many things; if Doctor Watson is any source to be trusted, one of those things is humble. Ignoring the fact that Holmes exhibits all the signs of a narcissistic personality
sure, okay, humble. Let's go with that.
Watson starts The Adventure of the Devil's Foot by explaining that Holmes has been far too unimpressed with fame and notoriety to allow Watson to publish any of his cases—you know, except for the other 63 ones he's published—but he decided, on a whim, that this story had to be told. This story, he explains in a telegram, is the strangest case he has ever handled.
Flashback to the spring of 1987. The boomboxes were filled with Bonos and Peter Gabriels, the streets rife with giant shoulder pads and larger hair, the... oh wait, that's 1897. My bad.
So spring of 1897, Holmes has a breakdown. He's taxed, overworked to the limit
and forced to go on a holiday. While vacationing in the Cornish countryside—yeah, I guess having a holiday in France wasn't invented yet—Holmes decides to spend his time researching some obscure remnants of the Cornish language.
I bet this guy is the most fun at parties.
So here's the breakdown: Holmes decides to be a goddamned archeologist, meets the amateur archeologist/vicar, who lodges the strange looking man named Mortimer Tregennis, who spends the whole story filing a name change. I mean, who happens to meet the doctor who is going to the house where the horrible murder happened. Do you get that? This is the sort of coincidental happening that Sherlock Holmes LIVES FOR
A woman is found dead, with her two brothers driven completely insane. They sat down to cards one night, and the next time anybody saw them they were driven horribly out of their minds, to the point of death. And in Cornwall, of all places! Won't someone think of the children.
There is one person left alive in this senseless tragedy—remember kids, card games always have four players!--and that is the third brother, Mortimer Trege... huh. And he's strange looking, you say? Oh yeah, he did it.
What? Fine, christ, I'll finish the story.
Holmes prowls around the house for a bit and asks a lot of senseless questions to everyone, and returns to his room. The next day a strange man enters! He's a distant relative of the family who also, as it turns out, was supposed to marry the now dead sister. Guess who's going to end up dead next?!
So the next day Mortimer is found dead, in the same way his siblings were found. Holmes prowls around some more, leaves a few cryptic clues for the local police, and goes back to his room to wait for them to find him. He waits for a while, because apparently the Cornish police don't give a fuck about no Sherlock Holmes up in their business. After getting tired of waiting
Holmes reveals that he totally jacked some substance from Mortimer's room, and offers it to Watson. I mean, I guess that it ends up being the hallucinogenic drug that was the murder weapon, but I'm not totally convinced that Holmes isn't just lighting shit on fire in the hopes that it'll get him high.
After nearly killing his long-time friend and companion, Holmes almost sort of apologizes, and they go to visit strange man. Strange man reveals all—some business about a root that he found in Africa that causes death. Deus ex machiroot. Holmes lets him off with a warning because he only killed for love, and a lovely vacation was had by all.