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 illegal time travel experiment threatens a war which 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.