Time travel novels are always fairly clear about one point, which is this: the fabric of space-time is not a thing to be trifled with. People can't change what's supposed to happen in history. When people attempt to change history, that's always when all hell breaks loose. Indeed, if people don't take care to protect the sanctity of the timeline, the whole universe could collapse into nothingness.
That's one theory, anyway. But it's not the premise of Time Tourists, a book in the Douglas Adams tradition of humourous science fiction and in the Harry Turtledove tradition of plausible alternative history. This book is based on a different premise. What would happen if people could go back in time and mess with everything - with no consequences?
