The story of The Tripods is a variation on post-apocalyptic literature, wherein humanity has been enslaved by "Tripods": gigantic three-legged walking machines, piloted by unseen alien entities (later identified as "Masters"). Human society is largely pastoral, with few habitations larger than villages, and what little industry exists is conducted under the watchful presence of the Tripods. Lifestyle is reminiscent of the Middle Ages, but small artefacts from the Modern Age are still used, such as watches.
Humans are controlled from the age of 14 by implants called "Caps", which suppress curiosity and creativity. Some people, whose minds are broken by the Caps, become vagrants. According to The City of Gold and Lead, Masters begin to believe that humans should be capped at an earlier age "because some humans, in the year or two before they are Capped, become rebellious and act against the masters"; but this cannot be done because Capping must wait until the braincase has stopped growing.