Sherlock has to help Randy, who thinks he might use again and is wondering if he should let his ex-girlfriend move back in, even though she’s the one who got him into drugs in the first place. Meanwhile, Joan looks through one of Sherlock’s old case files and finds evidence that leads her to a rock that leads to a dinosaur skeleton from Mongolia worth millions.
Joan and Sherlock follow the evidence and Gregson picks up a guy who admits to giving the rock to Newberg, but didn’t kill him. While they’re in interrogation, someone shows up with false credentials and takes the rock out of the evidence lock-up. Sherlock calls a friend in the auction business who leads them to the “Magpie,” a man who deals in the black market. But when they arrange a meeting, they find him dead and the rock containing the fossil destroyed.
Meanwhile, Sherlock tries being nice to Randy but it doesn't work, so he puts his foot down and tells Randy to get his ex-girlfriend out of his life. Randy responds by taking off.
Sherlock discovers a controversial theory called “Dead Clade Walking,” that claims dinosaurs were alive when science declares they couldn’t have been. He thinks someone killed for it, but DNA tests lead them to a man apparently being framed as he has a solid alibi. Joan and Sherlock follow the evidence to the museum and discover the curator, Dr. Thomas, was the culprit. He’d been trying to hide the fact that he’d been using black market parts for one of his displays.
Randy finally returns and admits that he got high with his ex the night before, but told her it was over. He isn’t sure if Sherlock still wants to be his sponsor, but Sherlock immediately offers to take him to a meeting.