Next Sherlock and Molly arrive at the flat of the man who owns the knit cap Mycroft and Sherlock used while doing deductions. His name is Howard Shilcott. Sherlock returns the hat and he and Molly follow the man inside. Howard works for the Tube and is a train enthusiast. He wipes the security footage after it's been cleared. He noticed a solitary passenger getting on at an empty car at one station, but when the train arrives at the next station, that car -- the last car -- is empty. This happened last week. What's more, the driver of that train hasn't been to work since. Howard says the driver's flatmate says he came into some money and is on holiday. Now, the train never stops. The doors won't open while it is operating and the maps show no other side tunnels or escape routes. Riddle us that, Sherlock! Sherlock watches the footage again and realizes he knows the lone passenger. It's one of his markers. We take a jarring trip through Sherlock's Mind Palace, which this week is more of a mind subway, then we cut to...
221B Baker Street. John stops outside the front door. He can't quite make himself go in yet, and more's the pity. One man bumps into him, and keeps moving without apology. This distracts John enough so that a second man has a chance to inject something into John's neck. John tries to fight off his assailant, but the drugs hit his system too quickly. The first man rounds back and helps the assailant lower John to the ground.
Back at Howard's place, Sherlock has figured out that the journey between the two train stations in question (Westminster to St. James Park) should only take five minutes, but on the night in question, it took ten. As Sherlock and Molly leave, he rattles off a list of things his going to need (old maps, etc.), then he asks Molly out for fish and chips. Molly asks him what today was all about -- them working together. Sherlock says it was his way of thanking her for everything she did for him. "Moriarty slipped up. He made a mistake, because the one person he thought didn't matter to me at all was the one person who mattered the most. You made it all possible, but you can't do this again, can you."
Molly stumbles over her response, but before she gets to a definitive refusal, Sherlock notices the diamond ring on her left hand and congratulates her on her engagement. Molly is quick to point out that her new man is not from work. They were introduced by friends and she's met his whole family, even his dog. Sherlock tells her he hopes she'll be very happy. "After all, not all the men you fall for can turn out to be sociopaths." Molly says, "No?" Sherlock answers, "No." He gives her a tender smile and an even more tender kiss on the cheek, then walks out ahead of her. Molly waits a moment. Her eyes glisten as she says more to herself than Sherlock, "Maybe it's just my type."