Briggs thinks it's weird that Holmes thinks he'd consider talking. Holmes calls him the greatest lock-picker in the world and offers him a consultancy, since he needs brilliant lock-pickers who are available. So: how did he get past the Leviathan door and who did he sell the secret to? Briggs says he just did the outer door and that Carter Averill was the one that knew a way past the ten-digit code. After establishing that Holmes will hire him for the nebulous consultancy whether or not his tip pans out, he says that he thinks Carter sold the secret to someone whose alias was Le Chevalier.
Outside the prison, Watson catches us up that Le Chevalier has stolen an original Shakespeare First Folio, Van Gogh's Pieta, and some Greek coins. But no one knows anything about him, so she thinks it's a fairy tale. This, in her opinion is a total snipe hunt. No one's ever caught Le Chevalier! But Sherlock Holmes has never gone after him.
Holmes and Watson go over the Pieta case. Watson says Le Chevalier's got style. Holmes agrees, because he's stealing things you can't just fence. As they study the records, he sees a picture of Peter Kent, leader of the fundraising drive that led to the Pieta acquisition. His cufflinks have silver tetragrams, which means they're made from Greek coins. The same ones Le Chevalier stole. Well, that was easy. And Peter Kent is in phone book! Well, the one on his phone.
Holmes and Watson have gone to The Kent Philanthropic Trust. The door-person is confused when they say they're here to see Peter Kent, so Holmes says to say it's about Greek Tetragrams. In the waiting room, Holmes gives a great sigh when Watson asks if something is an original Hopper. All of the paintings in this room original! And they're all very valuable. Except for one, which is a lithograph poster of the Pieta. Or... is it? Holmes goes over and breaks the glass on the lithograph's frame. He pulls out a knife and rips the poster apart to show the actual Pieta underneath. Nice work!
Peter Kent's son rushes in. Holmes says Peter stole the Pieta. And two nights ago, he stole the diamonds! Except that the son explains that his father had a stroke two years ago and hasn't moved since. Holmes tells Watson she was right: Briggs sent them on a snipe hunt. I like the idea that there was a legendary art thief with a fancy pseudonym baffling the police for decades, but it took Sherlock Holmes an hour to solve the case.
Holmes grouses to Watson about wild goose chases. Peter Kent's son didn't want all them to go public, so he let them take all of his father's loot, which they carried home on the subway. The word "court" makes Holmes realize that Carter Averill had a trial, so there ought to be some transcripts with at least some details of the crime. Maybe someone pieced it together! Watson thinks the only reason he's stuck on the copycat theory is because he couldn't do it himself and can't stand the idea that someone could be smarter than him. Well, wasn't Carter Averill able to crack the safe that baffles Holmes?