Will tonight be the first time anyone on this show actually says the word "elementary" for the first time? Let's find out!
In the cold open, someone breaks into a safe that's all fancy and electronic. At least two other black-clad people join in and they do the traditional move where you pull open a drawer and crudely throw the jewelry into a sack. That seems kind of sloppy to me, but I guess I don't know all the ins and outs of the jewel thief game.
In the brownstone, Watson comes down to a buzzing door. But before she can answer it, a pantsless lady named Gwen is there. And so is another pantsless lady named Olivia! They explain that they're here for Sherlock, and their "car service" will be here shortly. And there are crepes! Holmes comes down and tells Watson he was studying the differences between genetic duplicates, by which he means the Lynch twins. We never see them together, so there's a chance this is all an elaborate joke by Holmes.
Watson finally opens the door. A guy in a beard needs to talk to Sherlock. Holmes wants Watson to pretend he isn't home, but she's annoyed with him so they guy gets to come in.
He turns out to by Mr. Ehrlich, the president and chief engineer of "Casterly Rock Security," which is a Game of Thrones reference. Casterly Rock is the home of the Lannisters, who are most prominently known for always paying their debts and for having a brother and sister sleeping together. This security firm, however, makes bank vaults. In 2009, they introduced The Leviathan, which Holmes remembers as being advertised as "impregnable". The Lynch twins leave. It's a good thing that plot thread got resolved! Holmes says a Leviathan was broken into within a year of being launched, which you'd think would be a bad sign for an impregnable safe's long-term viability. Ehrlich says all four of the people involved were caught and convicted. But now the Swalbard Diamond Exchange was robbed last night. Holmes guesses that his job is to figure out how, since the police are concentrating on who. Ehrlich thinks there must have been a fifth conspirator in the original team who laid low until he could strike again, and Holmes says that's dumb. His working theory is that tf one group can break into it, so can a second.
At Swalbard, a feeb named David Batonvert says that Ehrlich's consultant can't see the vault until morning. Holmes says he's also an NYPD consultant, which he feels should get him into any crime scene he wants, and comments that David's name means "green stick". This baffles Batonvert for a moment, so Holmes strolls in, pointing out motion sensors and heat sensors and the like. The door on the safe is a foot thick, so it can't be picked. But maybe there could be a camera on a convenient fire extinguisher on the opposite wall? No, the police already checked that. So here's how the safe is opened: There's a ten-digit random code that changes every two minutes. One person has a fob that also shows whatever the number is, but the guy with the thing is in Gstaad. Really? There's only one person who can open the safe and you let him go to Switzerland? Holmes sits on the floor and stares at the door for awhile. Then he gets up and tries a couple of codes. They don't work.