The series is set at an unspecified time in the Earth's future. The world in the books mostly resembles the modern day world, with a few key differences. These differences are hinted at rather than explained outright. Firstly, most of the world's oil has been used up, and petrol cars are now only used by the very rich, elite and powerful. Aeroplanes are not flown at all – sea travel is once again the dominant form of overseas travel. The world (at least, the parts seen in Charlie's journey) now seems to run efficiently on solar and wind power.