is a series of young adult novels written by John Christopher, beginning in 1967.
The story of The Tripods is a variation on post-apocalyptic literature. Humanity has been conquered and enslaved by "the tripods", unseen alien entities (later identified as "Masters") who travel about in gigantic three-legged walking machines. 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 artifacts from later ages are still used, giving individuals and homes an anachronistic appearance.
Humans are controlled from the age of 14 by implants called "caps", which suppress curiosity and creativity and leave the recipient placid and docile, incapable of dissent. The caps cause them to worship the tripods. Some people, whose minds are broken (instead of successfully being controlled) under the pressure of the cap's hypnotic power become vagrants, who wander the countryside. One of the books contains a discussion among Masters that "We should cap humans sooner, to reduce the risk of precocious people getting independent-minded soon enough to try to evade being Capped, but we cannot, because we cannot Cap them until their braincases have stopped growing."