Time City is built on a patch of time and space outside history. It is full of wonders and haunted by "time ghosts," but it is nearly worn out and doomed to destruction.

In September 1939, Vivian Smith is on a train, being evacuated from London, when she is kidnapped by two boys from Time City, Jonathan and Sam. They mistakenly think she is the mysterious Time Lady disguised as a child. Only the Time Lady can wake the founder of the city, Faber John, from his age-long sleep, and only he can save the city.

Vivian wants to get home; Jonathan and Sam want her to help them in their quest through the ages of history to save Time City. Meanwhile, someone seems to be tampering with history, changing it over and over, complicating everything. When Faber John is at last aroused, Time City's and Vivian's dilemmas are resolved in ways that are as satisfying as they are unexpected.