Katherine only has a few hours to stop MEDUSA before London fires it at the Shield-Wall, but Bevis has a plan: a bomb.
He's going to make a bomb, and Katherine will sneak it in and blow up the computer that controls MEDUSA. Foolproof, right?
Well, a fool named Herbert Melliphant (you might remember him from the first chapter when he kicked Tom in the family jewels) overhears the whole plan and tells the Lord Mayor about it. Uh-oh, this isn't good.