After a time cut, Sherlock shows John Howard's security camera footage. It's Lord Moran who gets into and disappears from the train. Sherlock realizes the intel for which Mycroft's agent gave his life wasn't vague after all, but incredibly specific. "Not an underground network, John. It's an Underground network. Watching the footage again, Sherlock realizes that seven cars leave Westminster, while only six arrive at St. James Park. While the maps don't show another tunnel, there must be one, and that car must be in it. It's not long before they realize it's Guy Fawkes Day (which makes me wonder why the bonfire was the night before, but I haven't seen any Brits question that, so I'm sure it's something I don't understand). There's an all-night vote on a new anti-terrorism bill scheduled for tonight. Sherlock realizes Moran won't show. "Not tonight. Not the fifth of November." John chimes in with, "Remember, remember." Sherlock: "Gunpowder, treason and plot." Via Skype, Howard tells Sherlock he's found a never-opened station down by Sumatra road. There's a tunnel, tracks, platform and even staircases. The station itself was never built because of a legal dispute. Sherlock says, "It's right underneath the Palace of Westminster." John asks if there's a bomb down there. Sherlock walks off. John realizes the game is on and scurries after him.
The TV news airs a segment on the terrorism bill. Lord Moran watches from the safety of his hotel room. Meanwhile, Sherlock and John walk by Parliament and into Westminster Station. When John starts to call the police, Sherlock's voice is sharp as he tells him, "No!' He mutters about the cops always getting in the way. They enter a service gate and descend to the tunnels. When they arrive at the Sumatra Road Station, there's nothing there. Sherlock is confused. He enters his choo-choo Mind Palace again and imagines a fireball headed straight for him. Mentally, he moves to another spot along the line and envisions where the fireball would travel, which concludes with Sherlock envisioning an explosion that levels the Palace of Westminster. It's very V for Vendetta.
Back in the station Sherlock changes directions, hops down on the tracks and pooh-poohs John's concern about the live rails. On the way, they see demolition charges placed all along a vertical air shaft. Finally they find the errant train car. Explosives are packed under all the seats. Sherlock realizes the entire train is the bomb.