Out to the streets of London, which is a very pretty song. Holmes tells Watson about how London is riddled with CCTV cameras. He adds, "Now I know, for a fact, that some of London's network of watchmen verge on the competent." Which means that they know about the danger that 3D printers present. Looking up at a camera, Holmes holds up two handwritten signs: "Hello, I am Sherlock Holmes. I have a message for Langdale Pike. Need list of all 3D printers purchased past 24 months." Sherlock can't call him because Pike doesn't trust phones with GPSs. I don't think Mr. Pike is part of the official CCTV system, so it's not entirely clear why Holmes is using this technique to get in touch with him. Either the people who monitor the CCTV screens know how to get in touch with Langdale or Langdale himself is monitoring the screens.
Four hours later, Watson complains about standing around in a park waiting for a mysterious contact to bring them information. Sherlock says he's fine with Mycroft having retaliatory sex with her, but he warns her that she won't learn any childhood secrets, since he and Mycroft were packed off to different schools. A figure bumps her, and Watson suddenly has a list of 3D printer sales in her bag. Langdale Pike moves in mysterious ways. Holmes and Lestrade will investigate while Watson goes out with Mycroft. Sherlock wants to see who's right about Mycroft's intentions, adding, "My money's on me."
Mycroft welcomes Watson to his restaurant, which is vacant. He closed it so they could have a decent conversation. She tells him nothing's going to happen, and he sort of protests that it's not romantic. Although he doesn't say that exactly; he just asks if she thought it was supposed to be romantic. He says he lied when he claimed he lost weight through exercise. Watson interrupts that she knew he was sick. She saw scars on his wrists, which apparently were for "graft-versus-host." That means he had a bone marrow transplant in the last two years. He knows she's an ex-surgeon, so he's not surprised. He claims that his one regret is the state of his relationship with Sherlock. He wanted a way to make things better, but he fell into old habits as soon as he saw Sherlock. He butters Watson up by saying she's Sherlock's friend and asks her, "How does one become Sherlock Holmes's friend?"
Sherlock apologizes to Lestrade for not protecting him from the spotlight. But Lestrade says he loved it! Sherlock agrees that it was probably intoxicating. Lestrade: "I'm glad we helped so many people." Sherlock: "Well, I helped them. You only said you did." Dang. That's cold. Imagine what he was like before he became so courteous. Lestrade keeps studying the list of people who bought 3D printers. Holmes talks about making amends, and it's clear that he's going to try making amends to Lestrade for whatever he thinks he did to him. But before he can start, Lestrade notes that Nick Ginn (who he recognizes as Lawrence Pendry's handyman) bought a 3D printer a week before Mary Pendry died. He's very excited that this means he could be right, because he's more interested in being proved right than in justice.