By Montykins
Holmes is teaching Watson to be a detective! That mostly takes the form of making her guess what's going on before he corrects her. For example at the top of the episode, she thinks somebody shot two security guards. But what really happened is that they shot each other, because one of them was an impostor. He also sends her to a dry cleaner's several times until she deduces that they're involved in smuggling and human trafficking.
The main plot involves a rich philanthropist with a hereditary disease that causes dementia. He's convinced he's been poisoned, but nobody takes him seriously, since the disease is hereditary. And causes paranoia. But after he shoots his chauffeur, Holmes takes the case and starts talking about visiting Norwegian geneticists.
There's one lady geneticist who wrote a monograph Holmes owns. It's about the Warrior Gene, which indicates sociopaths. She sends a text suggesting that she has useful information, but she is inconveniently killed. This leads to a sidetrack where it could have been a guy who hated her and was known to get stabby. But it was really her fiancee, who went to the trouble of making blood with fake DNA markers to implicate the other guy. He did this because he thought she was cheating on him, but the name he thought he heard was really two giant charitable trusts.
When it turns out that those two trusts also had super-rich people recently struck down by the rare hereditary dementia, Holmes gets back on track with the regular case. After consulting with all the top geneticists in the world, he establishes that it's possible to create a poison that could cause it. He helps lead Watson to the conclusion that the only reason someone would be doing this is if they were trying to increase the amount of funding pointed toward curing this particular disease. So it's this one particular geneticist named Peter Watt. Case solved!