At a pet shop near the British Museum, the children discover their old friend the Psammead, caged and miserable. The children pool their pocket money together to rescue it, and in gratitude, the Psammead tells them to buy an amulet - or rather, half an amulet. Incomplete, the magic charm can take them to any place and time to search for its other half; but when the amulet is whole, it will have the power to give the children their hearts' desire.
In their quest, the children visit ancient Egypt, Atlantis and Babylon - they even meet Julius Caesar. But their adventures are not without danger: if they lose the amulet on their travels they will be lost in time, unable to ever return home.
