Watson kicks things off by digging through Holmes's big trunk of unsolved cases and settling on the murder of Doug Newberg. But the first Holmes knows about it is that there's a geologist named Gay in his house. Watson has spotted an unusual rock that has a white line on it that suggests that it's from the Cretaceous Era. So she and Gay go off to investigate and steal the rock in case it has a fossil in it. And it turns out to have a nanotyrannus, which is like a tiny baby T-Rex. It ought to be adorable, but it's really just a skeleton. And we don't even get to see the skeleton because it's a CT scan of a rock.
The next step is also due to Watson, who determines that Doug Newberg had a buddy named Diego who's almost certainly in the business of moving stolen goods around the city. When Diego's brought in for questioning, he seems pretty sincere when he denies killing his best friend. And while he's doing so, the fossil gets stolen from the evidence room. Did I mention that it's worth around $100 million? Well, it is. So you can see how people who get annoyed by it being stolen.
Holmes decides that it must be in the hands of an important fence. So he meets up with "C," a lady with whom he exchanges prurient correspondence. It's not strictly necessary to the plot, but it's a fun detour. C tells him of "The Magpie," so Holmes forges a rough draft of Martin Luther's 95 Theses to lure the Magpie out into the open. But when he and Watson get to the Magpie's place, he's been murdered. And the fossil has been destroyed!
Then the story takes a detour into paleontology nerdery. It seems that there's a theory called "Dead Clade Walking" that alleges that dinosaurs continued to live on after a giant asteroid or comet hit the planet, and it's very controversial. So Holmes decides that the fossil was destroyed because it could have been evidence for the theory. A bunch of prominent paleontologists are interviewed, and one of them has DNA that matches some found at the Magpie's murder. But he has an ally. It turns out that it was that guy's writing partner, who works at the New York museum and bought some dimetrodon bones from the Magpie. He was involved early in the episode to confirm the fossil.
The subplot involves Holmes attempting to mentor Randy. As you may be aware, no version of Sherlock Holmes is an entirely warm, open human being. But this one is trying to be better, and he honestly tries to help Randy with his problems. He doesn't know quite what to say, though. He finally recommends that Randy cut off ties with his drug-addict ex-girlfriend because she is not a healthy roommate. Randy doesn't take this advice well, and he leaves without a word. But he comes back at the end of the episode to admit that he got high, and Holmes immediately takes him to a meeting. It's a very low-stakes story involving an ancillary character, but I like what it showed us about Holmes's growth.
Holmes has an old-timey medical textbook and a drill. A text from Randy momentarily interrupts his foolishness, but he prepares to drill a hole in a mounted skull anyway. I will be honest with you: I would be happy with an entire episode where Holmes learns all about trepanation. But he's interrupted again by a text that says "Feel like I might get high".
Holmes goes into the living room to tell Watson where he's going, and he's surprised to see a woman he doesn't know. She introduces herself as Gay, and then clarifies that she is, in fact, gay. Wordplay! Holmes manages to tell Watson about Randy, and she's working on one of his cold cases. She's looking into the murder of Doug Newberg. The police never found a match for the DNA that was found at the scene, and Holmes thought it might have been a cuckolded husband. Watson is digging through Holmes's pictures of the scene, using black coffee to remove the black marker that he covered them with in a drugged-up haze. Gay is a geology professor who says that one rock has striations that suggest that it's interesting, so they're going to go check it out. Holmes wants to join them, but Watson reminds him about Randy. When you're a mentor, you have to interrupt your plans sometimes.
Holmes meets Randy in a diner. They have twenty minutes before the meeting, and Randy would like to talk to Holmes about his situation and state of mind. Holmes talks himself into listening. Randy's problem is his ex-girlfriend Eve, who wants him to help her get clean. She wants to live with him again, but she was around the last two times he got high. Holmes clarifies that Randy wants his advice and admits that he's familiar with "the allure of a dangerous woman." He recommends cutting her out of his life and turning his back on her entirely. Randy astutely asks if that's what Sherlock did when he was in a similar situation. Well, Sherlock's situation is a lot more complicated.
Gay is uncomfortable with prowling around in someone's backyard, but Watson says the homeowners aren't home. The rock is still in the backyard where it was sitting in the picture. Gay says the white lines mean it's very old marine limestone, probably from the cretaceous era. Watson takes a picture with her phone and Gay says the rock was probably cut out of the ground as an archaeological specimen. Holmes, via text, recommends stealing it.
Holmes meets Watson at the morgue. Gay decided not to get involved in stealing the rock, so she's not there. Randy is still sober. They're at the morgue because they have a CT scanner that they'll apparently run on demand. And the results are in! Looking at the computer screen, Holmes admits, "The dinosaur in Doug Newberg's backyard did indeed escape my notice." There's a little skeleton inside the rock!