When Bernice Summerfield visits the People - an incredibly advanced civilisation living in a Dyson sphere - she discovers that even in utopia they still have their problems.

An illiegal time-travel experiment threatens a war that could destroy them all. Rather than risk it, the People and their ultra-powerful computer, God, are prepared to eradicate the source of the problem - the ancient city of Babylon. But such action would involve the death of a quarter of a million human beings, and do incalculable damage to Earth's history.

Babylon - and the human race - have one hope, Benny returns to the cradle of civilisation to try and stop the interference. She has just one week to prevent a catastrophe that could mean she will never be born. Her only assistance comes from a Victorian linquist who has stumbled across the experiment himself. But he's no help at all - even thought he has a power neither of them suspects.