A Tale of Time City was first published in 1987 by British author Diana Wynne Jones. It tells the story of a girl, Vivian Smith, who is kidnapped while being evacuated from London during World War II and caught up in a struggle to preserve history. In this novel, Jones explains time travel with more reference to our current understanding of science than she does in many of her other works.
It is September, 1939, the start of World War Two, and Vivian Smith is being evacuated. On arriving at the station, she is kidnapped by two boys, Jonathan and Sam, and taken to Time City, which exists outside of what we know as History. Most of the plot takes place in Time City; the purpose of which is to oversee the course of history, and ensure that is stays on its "correct" path. To stop it straying from this path, the Time Police have Observers out in history, tweaking events to make sure that they go the right way.
Jonathan and Sam have kidnapped Vivian because they (incorrectly) believe that she is the "Time Lady," a legendary figure in Time City. The Time Lady is the consort of Faber John, another legendary figure. The legend states that at the end of history, Faber John and the Time Lady will return to Time City.
Sam and Jonathan believe that Vivian is the Time Lady because they overheard the Chronologue, powerful authorities in Time City, talking about history going wrong, and it being the fault of the Time Lady. They reasoned that, since she is Faber John's (in translation, John Smith's) wife, she must be calling herself Smith, and disguised as a young girl. Since Vivian fits these criteria, they had reasoned that she must be the Time Lady.
Vivian manages to convince them that she is not the Time Lady. However, the boys cannot return her to her own time, since the repeated use of the time-locks would be picked up on by the Time Police, and they would be found out. Vivian ends up staying with Jonathan's family, disguised as his cousin Vivian Lee. This works, because the real Vivian Lee is out in history with her parents, who are Observers.
Jonathan and Sam are especially concerned about the disruptions in history because it may mean that Time City will also break up with it. However, they then learn from Vivian's tutor, Doctor Wilander, that there is another legend about Faber John. This one states that Faber John created four Caskets - Gold, Silver, Iron and Lead - which, after being placed out in history, provide the power needed to keep Time City running. They also discover that the Caskets are hidden in what are known as the Unstable Eras - eras of history in which events are not fixed or stable and which might change at any moment. However, there are only three large Unstable Eras, so the fourth Casket must be hidden somewhere else.
Undeterred, Jonathan, Vivian, Sam and the android Elio go in search of the Caskets, using an ancient time-travel device they find in a secret room beneath a museum. They discover that the Iron Casket, which was hidden in Vivian's time, had already been stolen, and it was this that was causing the disturbances in history. They actually see it being stolen, but they are unable to catch the thief.
They fare no better with the Gold Casket; they find it, and its Guardian, but the Guardian refuses to hand it over, saying that at midday on the final day of Time City, he will come to the Gnomon Tower and return it. Although they try to convince him that they urgently need it, he refuses to hand it over, and promptly vanishes.
The Silver Casket is unfortunately hidden in the middle of the Mind Wars, but they still manage to find it, and the Silver Guardian, who, surprisingly, lets them have it. But, as it turns out, it is a fake, and the real Silver Casket had already been stolen, probably by the woman who posed as the Silver Guardian.
On returning to Time City, they see that things have gone terribly wrong. Since the Silver Casket had gone, history has gone into convulsions, and nothing is as it was supposed to be, with WW2 starting in 1937 and involving napalm and atom bombs from the start, and WW1 melding into the Boer War. In a final attempt to catch the thieves, Jonathan, Vivian and Sam return to the station where they had kidnapped Vivian, and where they are sure the thief must be. This goes badly wrong when they fail to catch the thief, cause an accidental explosion of a train carrying radioactive fuel, and return to Time City with two hundred evacuees in tow.
Because of this final disturbance of history, Time City has practically shut down. The Observers are being recalled from history, and this includes the real Vivian Lee and her family. But when they arrive, Vivian, Sam and Jonathan have a nasty surprise; the real Vivian Lee is the child thief that they saw stealing the Iron Casket, her mother is the false Silver Guardian, and her father is a man that they saw in the Age of Gold who tried to kill Jonathan. As Time City starts to fall to pieces around them, the Lees, Vivian, Sam and Jonathan make their way to the Gnomon Tower, for a climactic showdown that involves the return of Faber John, the awakening of the Time Lady, and a great deal of butter-pie...