Mycroft is in town! And Sherlock hates that! He's also bothered by what he perceives as tension between his brother and Watson, but he eventually gets Watson to admit that she had sex with Mycroft when they were in London. This gives him a chance to be a jerk to them some more. So that's fun.
Mycroft has brought his ex-fiancée Nigella, who Sherlock slept with to break them up. She married and then divorced a Marquess, so now she's in possession of an expensive stud horse and the title "Marchioness." But all is not sunshine and happiness in Nigella's life, because someone killed her lover and might also be trying to kill her horse. Sherlock quickly finds some fingerprints on a tree branch, which lead to the discovery that the Robles Cartel is involved somehow. Investigating the evidence left behind tells them that they're trying to kill Nigella, but they manage to save her precisely one second before a sniper kills her.
After a break in which Mycroft tries and fails to get Sherlock to eat a delicious dinner, Sherlock finds that Nigella's horse mated with a horse belonging to the head of the Robles Cartel, and the offspring got sold to a stable in Long Island. But the colt has markings that prove it's not the son of Nigella's expensive horse, because her horse is dead and she's been passing off fake horse sex. Then Holmes discovers that somebody who was a witness to a murder scene in Tampa is actually the notorious murderer El Mecanico. But his fingerprints don't match the ones at the scene, so there are still some things to work out.
After some cogitation, Sherlock decides that El Mecanico uses fake fingerprints when he's on a job. And the fingerprints he uses were once used at a liquor store robbery, which eventually means they belong to Strawdog Jed, a homeless guy who got his hands chopped off. So El Mecanico gets arrested and the cartel gets warned not to kill Nigella. Mycroft tells her she has to reimburse all the victims of her horse forgery, and she seems to agree. Then Sherlock has a coffee with his brother and it's not quite as awkward as it could have been.
An "open discussion" at a rehab meeting starts with crazy thoughts about disease. Holmes suggests that he's often thought that he should have been born in a different time. Then he introduces himself as Sherlock, an addict. But back to him: his senses are very keen, and this is an era of distraction. Don't I know it. He used to use drugs to dull his nerve endings, and now he sometimes wonders what he'd be like if he'd been born "when it was a little quieter out there." He might have been more focused and a more fully realized person. Not like Ancient Greece, because the dental care was awful. Someone suggests about two hundred years ago. It's Mycroft! He claims he's here to support his brother, and Sherlock melts down and walks out.
Sherlock stomps out, surprising Watson, and shouts that he wouldn't have said anything if he'd known the meeting was compromised. There's some awkwardness about Mycroft's presence, which is for a restaurant opening. It's the Diogenes, which is a reference to the club Mycroft is always in from the Arthur Conan Doyle stories. He's here early because of "circumstances." Someone both he and Sherlock knows is in trouble.
He takes Sherlock and Watson to the Diogenes, which will combine reservations and some walk-ups to keep things fresh and fun. Their mutual friend is a blonde who Sherlock is not happy to see, although she kisses him a few times. "Joan Watson, this is Nigella Mason. She's my brother's former fiance." The one who Sherlock slept with to prove...something. I forget what. Point is, he now finds it awkward to be around her and Mycroft, although they seem fine with it.
The chef explains that the plates are meant to be shared, but there's plenty of food. He knows that alcohol is off, but he asks if there are any other things he needs to be aware of. Sherlock informs him, "I lose my appetite in the company of social-climbing trollops." Watson gives him a Look. Sherlock is very angry that she's here, but she says she married Robert Suffolk, the Marquess of Loudwater. Sherlock clarifies that it's between an Earl and a Duke. The Marquess was gay, so she turned to Dalton, the man in charge of his stable of racehorses, and there was a divorce. But she's still a Marchioness. She got to keep Silver Blaze, a horse she loved. And there's a demand for his services. Sherlock, still very stiff, says, "So you've become a horse pimp. How fitting. Must be lucrative!"
Silver Blaze is in Ulster County, and when Dalton was going to administer "the usual medication," a man surprised him and shot him. The murderer left behind a bag of supplies, including a syringe of potassium, possibly to kill Silver Blaze. She wants justice for Dalton, although Sherlock thinks it's to protect her meal ticket. She hopes Sherlock can do this, because Mycroft forgave her when he fell sick. This draws a look from Sherlock, who is just now learning about the leukemia. "I'm sincerely sorry for your trouble. But. This. Is an awful toxic creature." He leaves.