Micah Erlich hires Holmes to solve urgently the robbery of diamonds in David Batonvert's Manhattan exchange from his world-reputed security firm's top-range vault model, the 'impregnable' Leviathan, before the news wrecks the manufacturer. Both men remember it was actually cracked once four years ago by a brilliant team of four, who were tried and jailed, but draw different conclusions concerning a connection, which Holmes has to revise while his investigation progresses with the help of his own consultants. Meanwhile he helps Watson deal with her unexpectedly visiting family rather then cancel her buddy job with him.
- "Elementary" - "The Leviathan" - Dec. 13, 2012
We see four masked thieves break into a high tech vault and steal lots and lots of shiny jewelry.
This leads directly to Sherlock getting a visit-- just as he's saying goodbye to two comely sisters-- from the head of the company that made the vault-- called the Leviathan. He is desperate to figure out how it was cracked. Not only is the diamond company that was robbed down $40 million but if word gets out that the safe company's most famous safe has been cracked, again, then their reputation will go down the toilet. He's heard Sherlock might be able to help.
While Sherlock heads down to check out the vault, Watson goes to meet her mom or brunch. Sherlock notes that she looks like she's heading off to a job interview not meet someone she loves. We soon learn that, although Joan probably loves her mother, that mom is maybe not super psyched that her daughter stopped being a doctor and started being a "babysitter" to drug addicts. We learn that Joan's brother Oren will be in town the next night for dinner and that they should all get together because he's finally going to introduce them to his girlfriend.
Sherlock is at the vault running through a million ideas, noting all the safety gizmos, cameras, and redundancies and it's driving him mad as he talks to the vault guy and meets the diamond company lackey, a man whose surname means "green stick" in French. Sherlock is so mad with deduction that Watson comes down to get him pointing out he's been there all day and that this kind of obsessive behavior is very much like a dry drunk. He stays a little longer, until she actually falls asleep. He's stuck on the idea that the computer programmed passcode was updated every two minutes randomly and the person who had the fob with the passcode was unreachable in Gstaad. He eventually gets so frustrated he just grabs an axe and smashes the glass displaying the code above the keypad.
They decide to try to speak to one of the original crew who cracked the Leviathan under the reasoning that the only way it could be cracked was by this crew selling their secret. The mastermind behind the plan died in prison but one member of the crew says he heard he did sell the secret to a man called Le Chevalier. Apparently, he was a wily master thief who stole priceless things that would be unsellable on the black market. Sherlock makes short work of finding the man. Unfortunately, he's not their guy since he recently had a stroke and can't walk. They do find his booty, however, and return it to Capt. Gregson. (All but one famous Van Gogh painting that Sherlock wants to keep for just a little while.)
Joan heads off to dinner with her family to discover Sherlock is already there. He charms the pants off everyone and explains how instrumental Joan has been in not only helping with his sobriety but solving crime and saving his life and Joan's mother sees that her new job is important. Later when Joan thanks Sherlock he waves it off saying Joan's family is conventional and he knew what to do to impress them on her behalf and that she is unconventional. It's the closest he's gotten to saying how much he appreciates her.
Sherlock realizes that one of the original thieves didn't settle so there was a court case. He reads the transcripts and sets his sights on the jury. Four members of the jury match up with the skills of the original four man robbery crew. One of them is related to the Green Stick man from the diamond company. When one of the new quartet turns up dead, Sherlock deduces the diamonds were at his house in a decorative glass container and that one of his crew mates killed him out of greed. Another turns up dead.
They call in the whole jury from the original case to get DNA. The two living members of the crew who do one seems reluctant at first to give them the swab but does eventually. But then there is no DNA match and they're perplexed. Until they find out he was a bone marrow recipient, which can change the recipients DNA make-up vis a vis a saliva sample but not a blood sample. When he balks at giving a blood sample, Sherlock lays it out and gets his man.
Later Joan's mom shows up to apologize and be nice to Joan and say she saw a spark in her eyes when she talked about working with Sherlock and hints that she should stay with him and help him.
Sherlock returns the painting.