Everyone over the age of 14 has been consumed by a virus that essentially turns them into zombies. Only the children are immune...for now, maybe.This is number four in what Charlie Higson is now planning to make a seven book epic, and there's a sense of things being set up for the later books, especially in the later chapters. There's a change in the behaviour of the adults, for good and bad - the good guys get an ally, and the bad guys get a leader.This is a book without fault. There isn't a single wasted character or event, no matter how minor, and all the strands of plot tie up at the end and then leave room for more books (apart from DogNut, who I'm sure will appear in Book Five or somewhere down the line...).What's getting hard after four books and a gap of a few years between them, is to get the timeline sorted out. Events in this book overlap events in the other three, and it's hard to remember who all the characters are in the previous books and their 'status' in this one. But that's a minor niggle.Higson goes to lengths to point out that the monsters inside - the children who decide to lead the children - are as dangerous as those outside. There are shades of Lord of The Flies in Ed and Little Sam and the situations they find themselves in, and I think the comparison is a worthy one.This is not a book for the squeamish. A nine year old boy gets flogged, anyone can die (and they do), and the fights against the adults are long, bloody and vicious. It doesn't go into extravagant details, but it doesn't shy away from them either. Be warned: This is the book Stephen King would write if he wrote YA.The real star of the show are Little Sam and the delightfully batty (or is he?) The Kid, who talks like Alex from A Clockwork Orange but is sharp as a sat-on-box-of-pins. Sam's grim determination to find his sister and later The Kid is one of the underlying themes of the book - and there are so many: dictatorships, loyalty, sacrifice, friendship, not judging people by their appearances, rebuilding society. Everything is packed in there, but nothing feels rushed or thrown in. This is a book carefully constructed to make you think and reconsider, and I'm already hungry for the next three sequels.