The episode begins with few masked men breaking into a vault and stealing a lot of expensive jewelry. Next, the doorbell is ringing and Watson comes downstairs to get the door. But she is surprised to see Gwen and Olivia in the house and they are barely dressed. They are twins and they spent the night with Holmes. Holmes arrives and he tells Watson that he shares a mutually beneficial relationship with the Lynch sisters. Watson finally opens the door and a man asks for Holmes and tells her that he was referred to Holmes by one of his friends in England.
The man who has come to meet Holmes is Micah Erlich and he is the head engineer for The Casterly Rock securities, and this company is famous for making unbreakable vaults. Holmes is aware of their works. Micah then tells him about their latest vault that was made in ’09, called the Leviathan and they had termed it impregnable. Micah says that their vault was once broken and that the four men were brilliant; but none of them talked. But now once again four men have robbed a place and it is impossible that such a brilliant team was put together again. Micah the tells Holmes that a diamond exchange was robbed last night, and they are missing 40 million worth of stones which were protected by his vault. Micah fears that if people know that another one of the Leviathans has been robbed, they will be doomed. Holmes tells him to take him to the exchange.
At the exchange, Holmes meets David Batonvert, the floor manager who thinks that Holmes might not be able to deal with this. Holmes then looks at the security measures and tells them the ways that they can be tackled. They finally reach the vault which needs a foot long key to be opened. Also there is a 10 digit code that keeps changing every two minutes and the code is now with the owner who is in another country. Holmes tells them that he this will take time. Watson arrives. Holmes has been in the vault for 17 hours. The code system amazing as it has an ocean of randomness which makes it difficult to predict the code. He then smashes a box, takes out the axe and then axes the keypad. At home, Holmes tells Watson that they need to know who broke into the vault if they need to learn how it was done.
Micah had earlier told Holmes that there could be a possible fifth conspirator in the team who got through with the first robbery. But Holmes thinks that it is necessary as the original team could have also passed on the secret to the new team. Carter who organized the crime, died in prison last year and the other three have still not responded to his request to meet. Watson leaves to meet her mother for brunch. Her mother tells her that her brother is coming home along with Gabrielle. Holmes calls Watson and tells her that Charles Briggs who was from the original team has agreed to meet them. They meet Charles. Holmes tells him that he collects expertise from around the world for his consultancy work and he is ready to hire Charles.
Charles tells him that Carter was approached by a man while he was sick and he wanted to know how they broke into the vault. He was ready to pay and Carter thought it was the best way to get some money for his family before he died. Charles tells him that he does not know how they got in as he was only responsible for getting the team past the outer door and Carter broke the code and never told the rest of them. Charles only knows the alias the thief went by, the famous Chevalier. They then look at files and the items that the thief stole. Holmes finds out that a man named Peter Kent, who headed the Kent trust, was wearing cufflinks which resembled the coins that the thief stole years ago from the museum.
They go to visit Peter Kent. They find a replica of Van Gogh’s Pieta and Holmes rips the painting to reveal the original Pieta hidden beneath it. Peter Kent is the famous thief who approached Carter. But Peter’s son tells them that Peter had a stroke two years ago and hasn’t moved ever since. The recent robbery wasn’t him. Holmes thinks that Charles sent them on a snipe hunt. They return the stolen artifacts to Gregson and Holmes doesn’t want him to ask too many questions as he does not want to send a sick and dying man to prison. Holmes keeps the original painting of the Pieta with him and he is going to read the trial transcripts of the case.
He realizes that the three of them had opted for plea bargain while Carter tried his luck at a trial. He thinks that someone would have put the puzzle together. Oren calls Watson and tells her that he is excited to meet her and her client. Looks like Holmes messaged Oren from Watson’s phone and told him that Watson would be attending lunch with her client. The next they go to a place where they keep exhibits and they find a coffee order that was placed by the former team. Holmes wonders why the jury checked the coffee order thrice. There is a code written in a programming language that is impenetrable.
His contact in London sends him the decoded format. Holmes thinks that someone from the jury figured out that this is a programming code and used it to conduct the heist. They check the list of jury and find that Justin Guthery was a software engineer and that he is the one who stole the diamonds. They go to Justin’s house and find out that he is dead; he jumped out of the window. Gregson arrives and Holmes points out to blood on the floor and the splatter pattern looks like someone was hot on the nose. He then points out to three vases filled with rough looking rocks and Holmes feels that the person who is responsible for Justin’s death knew that and he took the stones with him. But he leaves a piece of rock behind in the vase in a hurry. This is now a murder investigation.
Next, Holmes meets Watson’s family and defends her choice of occupation. Watson thanks him. Holmes now wonders who would have Justin collaborated with for the heist. Holmes had flicked Justin’s phone from the scene and he found three names and numbers saved on the phone. After pondering for a while, he deduces that Justin used three members from the jury to perform the heist; Alex, Amilie and Jeremy. Amilie’s maiden name is Batonvert, and she is David Batonvert’s sister. And one of them is killing the other to keep the goods. Gregson tells Holmes that the only juror that they could find is the engineer Alex. Holmes suspects Jeremy to be Justin’s killer as Amilie cannot throw Justin out of the window, but as Jeremy readily gave his DNA sample, they go looking for Alex.
They then find that Alex is dead too and he was killed two days ago. Gregson tells Holmes that the blood found in Justin’s house belonged to an army chaplain named Audrey and she is deployed to Kabul a few months back. Watson finds out that Audrey’s sister died of leukemia and Audrey did everything to help the sick since that time and they find a bone marrow donor batch in the house. Next, we see that Jeremy is in the interrogation room. They want his blood sample. Jeremy had leukemia but was cured with bone marrow transplant. So, while his saliva, his fingerprints would be his own, his blood would carry the DNA of his donor. Audrey was Jeremy’s donor. Jeremy killed Alex and Justin. Case Closed.
Next, Watson’s mother visits her and tells her that she knows that Watson likes what Holmes does and indirectly hints that she should pursue her liking as it is said that people find their paths in the strangest of ways. Holmes arrives and turns on the news; Pieta is returned to the museum. The episode ends.