Word travels fast! Cut to King's Landing, where Cersei is asking Tyrion who killed Renly. Tyrion says that accounts differ, but that most people blame Catelyn Stark somehow. Which makes sense, because only Renly, Catelyn, and Brienne saw it happen. The next person into that tent is just going to see Renly and three guards dead, with Catelyn and Brienne gone. There are other silly theories that Tyrion has heard. Cersei is delighted by this turn of events, because it's always good news when the leader of an enemy army suddenly drops dead. Tyrion says that according to Varys, Renly's men are flocking to Stannis, with the result that Stannis now has a giant army. Cersei says that Littlefinger says they can outspend Stannis three to one. Well, that's great. When that giant army knocks on the front door of King's Landing, I guess you can just flash your bankroll at them. Maybe get a gold-plated welcome mat. For the record, Tyrion agrees with me that their primary concern is the giant army.
Cersei changes the subject to Myrcella being packed off to Dorne, which Tyrion claims will be safer for her. Cersei doesn't think Tyrion's primary concern is Myrcella's safety. She appears to just be assuming that Tyrion is getting rid of Myrcella just to be a jerk. The truth is that he wanted to find out who was loyal to Cersei, and he was willing to sell Myrcella out to do it. Remember that one of the options was to marry her off to creepy Robin Arryn. Tyrion blandly says that Myrcella is a "Sweet, innocent girl". Cersei is mad at Tyrion for being clever with his schemes and plots. Tyrion corrects her: "Schemes and plots are the same thing."
It's time for another subject change, back to the topic of that giant army that's coming for them. Cersei says the King is taking personal charge of the defenses of King's Landing. Oh good! I'm sure a sadistic sociopath in his mid-teens will be perfect for planning the defense of a city. Tyrion would like some details, but no dice: "It's the king's royal prerogative to withhold sensitive information from his counselors." Cersei is smug. Tyrion leaves.
Bronn keeps guard while Tyrion and Lancel Lannister are in a secret crate. Well, really it's a palanquin or sedan chair. That's one of those enclosed chairs you ride around in while your muscular servants carry you. It's just sitting on the ground on a King's Landing street, so it's essentially a crate right now. Okay? Lancel tells Tyrion that Cersei's secret weapon is something called "wildfire." And Lancel promises that of course he'd never lie to Tyrion. Tyrion asks Lancel to speculate on the vile allegations about Jaime and Cersei, and whether he thinks they'd make Jaime more or less likely to kill him. "When I tell him you're fucking her, I mean." Lancel correctly interprets this as an accusation of falsehood and insists that the alchemist's guild has thousands of pots of wildfire that they're preparing to launch at Stannis's ships. He heard Cersei talking. He offers to swear on his life, but Tyrion doesn't care about Lancel's life. Lancel launches into a more complicated oath, which Tyrion finds either convincing or too boring to listen to. Lancel exits the palanquin. Tyrion asks Lancel to tell Bronn to kill him if anything should happen to him. He does: "Please kill me if anything should happen to Lord Tyrion." Obedient, isn't he? I can see why Cersei likes having him around. Sometimes you're in the mood for someone who's easy to boss around. Anyway, Bronn cheerfully agrees as Lancel scurries off.