The doorbell rings and Watson answers.
At the door is a Mr. Erlich, president of Casterly Rock Security, who wants to hire Holmes. Casterly created an “impregnable” vault called the Leviathan in 2009, and it was broken within the year by four master criminals. All four were caught and questioned, but admitted nothing. A similar vault (holding diamonds) has been broken and Erlich wants to find the thieves before word gets out and Casterly is ruined.
Holmes accompanies Erlich to the vault and studies it under the eye of prickly vault manager Batonverdt. Holmes, with typical “look how much I know!” quirkiness: “Your name means ‘green stick’ in French!”
Holmes says finding out who did it will help them find out how they did it. “When you rule out the impossible, whatever remains, however improbable, is the truth.”
Watson meets Holmes at Sing-Sing to interview Charles Briggs, one of the original four master criminals. He was responsible for picking the outer door’s lock, and says the other three had specialized tasks too. He points them to “Le Chevalier”, an art thief.
They find Peter Kent, art collector and philanthropist, and prove he’s Le Chevalier. However, he had a stroke 18 months ago and couldn’t have broken the vault recently. They take his stolen art and return it to Gregson, who grudgingly accepts their story about “just finding them”.
Holmes still believes the recent break-in was a copycat theft. Watson suggests reading court transcripts from the original criminals’ trials.
Evidence from the case – a paper scrap with all four men’s names on it –has an obscure programming language on the back. The random number generator (one of the vault’s locks) was cracked with this program. Justin Guthrie, a jury member, was a software engineer and recognized the language. Holmes suspects he was the fifth team member.
Watson’s been roped into dinner with her family by Holmes, who hijacked her phone. Holmes shows up and charms everyone, then defends Watson’s job to her family. “You can measure her success in careers restored.”
Guthrie is found dead, but Holmes says it wasn’t suicide – someone killed Guthrie and took the diamonds. He searches Guthrie’s phone and finds names of three other jury members. Their careers match the skillset of the original criminals. One was Batonverdt’s sister.
The original jury is assembled for DNA sample collection, to find Guthrie’s murderer – there was blood found in Guthrie’s apartment.
The blood is traced to Audrey Higuerra, but it turns out she was a blood marrow donor. A leukemia patient received the marrow, meaning that patient’s blood was in Guthrie’s apartment, not Higuerra’s blood.
The recent break-in was the result of a copycat team of people from the jury at the original trials. Guthrie was murdered for his share of the diamonds. Holmes was right. Case closed.
Watson’s mother visits to tell Watson that she disapproves of Watson being a sober partner because “it never seems to make you happy.” She says that ‘sober partner’ seemed like something Watson picked “out of a sense of duty”. However, she says Watson seems excited about working with Holmes and being a detective.
Watson says her time with Holmes is up soon, and “I’m not a detective”. But her mother cautions her not to give up so easily. “People find their paths in the strangest of ways.”