Mycroft is taking her to dinner, possibly at his restaurant that has two Michelin stars. Sherlock says Mycroft clearly intends to bed her as revenge. He alleges that she's attracted to Mycroft because she can't sleep with Sherlock, and Mycroft is a "cheap knockoff version." To get out of the conversation, Watson looks at the crime scene pictures. Sherlock declares that one of the masks on the wall in a picture is out of line. But they were perfectly lined up before Mary died, because Mary conveniently took a picture with them in the background. This show relies really heavily on the idea that people are constantly taking pictures that happen to show the alignment of their furniture and decorations. Holmes wants to investigate, but he has to do it under the aegis of the Lestrade investigation. So he plans to tell Lawrence he found a threatening suicide note from Lestrade. Which he now makes Lestrade write.
Lawrence reads the note, which is all about how Lestrade is going to kill Lawrence and then himself, so he can die a true policeman. Holmes recommends changing his routes to his office. Lawrence doesn't want to invite more police into his home, because he's already trying to protect himself from one rogue police officer. Holmes claims Watson is America's foremost expert on home security and gets Lawrence to let her look around the house. There are motion detectors and security nodes on the shatterproof windows. Lawrence mentions the security nodes again, but I'm no closer to knowing what those are. While Lawrence is distracted with his nodes, Holmes looks at the mask, and a nail falls out of the wall. Time to go! Once they're outside, Watson complains about having to be the number one security expert in America, because that's more pressure than being number twenty or so. Holmes ignores her so he can declare, "Lestrade was right. Lawrence Pendry did kill his wife. I know exactly how he did it."
Back to the theater. Holmes recaps the case. In his opinion, the murder weapon was in the kitchen when the police were there. It was a white plastic gun that Pendry melted with acetone. By the time the police arrived, the gun dissolved into a white milk-like liquid. Watson explains that it's from 3D printing, which can make a perfect gun that needs only one piece of metal: a nail to act as a firing pin. Holmes says the tip of the nail he found was slightly charred, which he attributes to "carbon scoring from where it struck the bullet." Lestrade wants to take this to Hopkins, and Holmes reminds him their actual evidence is pretty flimsy. They need to find out how and where he made that gun, so then they'll have something concrete. I mean, not that the gun will be concrete. That would be weird.