Those adults who didn't die became "grownups"; zombies who smell, look gross, and wander around London, looking for food. They're a nasty bunch, and the Holloway kids have learned to defend themselves against the grownups. Slowly, however, they are being picked off, and when Jester arrives at their headquarters, telling them about another group of kids living at Buckingham Palace, they decided to follow him there. Walking across London should seem fairly easy with no traffic to contend with, but the kids soon find grownups gathering to stop them.
All of the characters in this novel are well developed and each shows a strength of character that momentarily stops you from realizing they aren't adults--just kids struggling to survive with no answers to what happened, and no way to know what will happen in the future. My favorite character is Small Sam. He's a small in stature, but incredibly resourceful and clever as he is taken by the grownups in the opening pages of the novel. Small Sam is terrified, but manages to keep his fear at bay as he works to escape and rejoin his sister and the others.
I'm looking forward to the sequel--there has to be one! Read this, and recommend it to your favorite teens.