A sickness has afflicted everyone over the age of 16; anyone who is a "grown-up" has become a decomposing, brainless creature that survives by feeding on children. The children and teens have barricaded themselves in fortified buildings, fighting off attacks from the grown-ups who travel in packs, like hungry dogs. Before long, the young survivors are promised a safe haven in Buckingham Palace and make their way to it, crossing London on a perilous journey that will test them in harrowing ways. But their fight to stay alive is far from over—the threat from within is as real as the one on the outside.