Dinah, Lloyd and Harvey thought the Demon Headmaster was gone for good, so when they move back to their old house everything seems safe at first. Then strange things start to happen. Libraries are closing down, the internet keeps crashing, and telephones refuse to work. When people start to wear matching badges with a grabbing green hand, and begin speaking robotically, Dinah and the boys know they need to find out what's going on. Their search leads them to the Hyperbrain, a fantastic new computer invention that accesses the human brain to compile its data. But why has its inventor started to act so strangely, and who is the mysterious controller at the Artificial Intelligence Unit? It sounds like it could be the Demon Headmaster. But surely that's impossible . . . isn't it?