So Emil Kurtz, who was Watson's mole inside Morland Holmes' organization, was murdered in what was made to look like a café robbery gone bad. But Watson knows better, and when Sherlock confronts her with her recent bizarre behavior, she confesses to having double-crossed his father.
Everything then points to Morland having offed Emil after discovering Emil was spilling secrets [to Watson, though whether Morland knew it was Watson was still a question mark].
Still, it's a given that Morland himself didn't walk into that café and shoot anyone. So the first step is to find out who did and then trace it back to Daddy. This is achieved via a very strange coincidence (if that's the word for it)—one of the victims of the shooting wasn't shot; she had an acute allergic reaction and died from that. Her sister, who was also at the café, then tells a story of that having happened only once before, and doctors determining her sister was allergic to mountain lions. Everyone now: The fuck?
Okay, so the thought is that the murderer had been near a mountain lion. None of the zoos have mountain lions. That leaves taxidermists. And that's how they find Arthur Tetch, who readily admits to having been approached and eventually hired to murder not only Emil but some research lady some weeks before. The man who hired him called himself Mr. King, but Tetch's description places the guy as too young to be Morland (and surely Morland would keep his distance from something so distasteful anyway). One of Morland's lackeys then?
Sherlock finds surveillance equipment in Emil's apartment, too, seemingly recently installed.
Anyway, the story wends through the research lady having been doing work on having people take an online survey to discern whether they were psychopaths, sociopaths, etc. I would argue that, while it's fair to assume a wide pool of responders, they're also going to be largely self-selecting and you're going to get skewed results, but whatever. It's pretty clear that someone on this research staff was using the responses to pinpoint people like Tetch who might be willing to kill for money.
Meanwhile, Morland stands clear of wrongdoing. He admits to some more shady dealings but nothing approaching murder. And Sherlock is forced to admit, in turn, that he'd jumped to conclusions—something he usually prides himself on not doing.
As things wind down, it's the nicknames of the people doing the hiring and murdering that sparks Sherlock in another direction—he recognizes a pattern in the naming convention that leads him to believe Moriarty is back.
DUN DUN DUN
They've been teasing this for weeks, so we're less impressed than we might otherwise have been.
On a softer note, last week Morland had come to the brownstone in search of something he'd lost. Turns out it was a ring. A ring that had belonged to Sherlock's mother, in fact, that Morland planned to use as a kind of bribe to some countess. Sherlock had discovered the ring back when he'd moved in, and had secreted it in the fireplace. He doesn't tell his father that, though.
One has to wonder whether Morland was lying about Sherlock's mother being an addict. I wouldn't put it past him, really. The man manipulates, and that's a strong form of emotional manipulation right there. Maybe it doesn't matter, but it's the kind of thing that interests me—the psychology of characters and their motivations. I'm a character person first and foremost, which is pretty obvious if you read any of my books. (Peter in particular has been lauded as a great character study. Give it a whirl, eh?)