- A wealthy scientist comes to Holmes with a unique case: he believes he has been "poisoned" with a genetic disorder. Holmes has to determine how a healthy person can be contaminated with a genetic disease that no one in his family has.
- Gerald Lydon hires Holmes to check his extraordinary theory: being 'infected' with a rare and fatal genetic disease. Shortly after, Gerald's driver and henchman Crabtree is murdered. Consulting genetic researchers, Holmes learns genetic 'poisoning' is possible; the rare disease occurs improbably often with wealthy research donors but requires an expert. Meanwhile trainee detective Watson wrestles with the case of Holmes' Polish dry-cleaner Agnieszka.—KGF Vissers
- Watson, Holmes, and Bell are surveilling a crime scene that appears to be two dead museum security guards. Since Sherlock has been mentoring Watson about detective work he gives her a crack at it. She does the best she can but is wrong and Sherlock explains how. He tells her not to let failure discourage her since deduction is not just a skill it's a point of view.
They are approached by a limo driver named Crabtree. He works for Gerald Lydon, a muckety muck rich dude who owns 18 patents. They go for a ride with him and his assistant/nurse. He explains that he has dementia due to a rare genetic disorder, CAA, that will eventually kill him. He strongly believes he was given the disease by someone and therefore it's murder. He asks Sherlock to take his case. Sherlock expresses his condolences about his condition but since it's hereditary Sherlock refuses.
Later Crabtree delivers a bee in a box. It is a rare, nearly extinct variety and meant as a bribe from Lydon. While Sherlock is intrigued he declines convincing Crabtree Sherlock is a man of conviction. Not that this conviction helps much since Lydon then shoots Crabtree. When Gregson and Bell arrive, Lydon calls for Sherlock and again asks for help. This time Sherlock agrees...and takes the bee.
Sherlock and Watson go to a genetics lab to confirm his diagnosis and meet Natasha Kadmon, who wrote her dissertation on "warrior gene" i.e. the "sociopath gene." She knows Lydon thinks he was given the disease. She says that maybe there are seven people who might be able to crack the ability to give it to someone.
Sherlock and Watson set some ground rules on chores and he sends her to a dry cleaners and agrees to clean the fridge.
While they research geneticists crafty enough to give someone a hereditary disease, they get a text from a blocked number with a picture of a molecule and a note saying they are onto something. They realize it's Natasha and set up a meeting. When they arrive, she is dead. Stabbed to death.
Sherlock thinks it's related to Lydon but Natasha's fiance, also a geneticist, arrives and points a finger at a recently released convict named Benny Cordero, a man who claims she used his blood without permission in a study. He thought signing up would win him brownie points with the parole board but when she discovered he had the sociopath gene it wasn't really useful. Sherlock insists that Benny is innocent.
Watson finally goes to the dry cleaners and is served by an angry Polish woman working there who doesn't know what to charge and doesn't want to give her a receipt.
Sherlock and Watson go see Lydon's sons, people who stand to gain from their father's death and each give a DNA sample.
Cordero's blood is a match to Natasha's crime scene but he says it's not possible because he was busy taking blackmail pictures of his neighbor and his nanny.
Watson goes back to the dry cleaners and starts picking up weird clues that something is amiss, including very expensive security cameras. She realizes Sherlock wanted her to notice these things and solve the puzzle.
After they run a DNA test on Cordero and it matches, Watson and Holmes realize that someone, likely a very skilled geneticist, was able to fake a DNA sample. Turns out that her fiance killed her. After they were engaged she discovered that he himself had the sociopath gene and started pulling away from him so he killed her and faked the DNA sample. (Thereby sort of proving the point about the gene.)
He thought she was cheating on him with someone named Lincoln Dunwoody but it turns out that she wasn't. Lincoln and Dunwoody were two separate people, wealthy philanthropists who also mysteriously came down with the same rare disease Lydon had even though they had no family history of the disease either. It turns out Natasha was a whistle blower. She realized someone was poisoning wealthy people with the disease so they would give money to research for its cure. The culprit? Her boss at the genetics lab who, no surprise has the disease himself.
Watson goes back to the dry cleaner with Bell, having figured out that they were fronting a money laundering scheme, and has the angry woman and her enforcer arrested. Sherlock congratulates her on a detective job well done.
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