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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. People who are capped are happy to leave home and serve the tripods. The caps cause them to follow the Tripods' will. 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."
The White Mountains (1967) 
In the future, the world is controlled by machines called Tripods. Life goes on largely as it had in the pre-industrial era, as all of humanity is subject to mental controls which prevent anyone from challenging the established order. Will, a thirteen-year-old living in the small (fictional) English village of Wherton, is looking forward to the transition to adulthood which will take place on the next "Capping Day", until a chance meeting with a mysterious Vagrant named Ozymandias sends him on a quest to discover a world beyond the Tripods' control. He is accompanied by his cousin Henry, and a French teenager named Jean-Paul, nicknamed "Beanpole" for his height and slimness, and punning similarity to his real name.
On their journey the boys have many adventures. The novel climaxes with Henry and Beanpole discovering that earlier, unbeknownst to them, Will was captured by a Tripod and implanted with a tracking device. They realise the tripods intend for the boys to lead them to the human resistance (an organisation which the boys have been seeking). Henry and Beanpole remove the device which causes a nearby Tripod to attack the group. The boys defeat the Tripod and eventually reach, and offer their services to, the resistance, located in the titular White Mountains.
The City of Gold and Lead (1968) 
Will, Henry, and Beanpole have spent a year living among the free men in the White Mountains. The Resistance now charges Will, Beanpole and a German boy, Fritz, now wearing realistic-looking yet harmless caps, to infiltrate a Tripod city by competing in a regional sporting exhibition: the winners of the events are always offered to the Tripods for service. Will, a boxer, and Fritz, a runner, win their respective contests, while Beanpole fails to win in the jumping events.
The winners are taken by Tripods, which they discover to be machines operated by living creatures, to the Tripod city, which is located in a sealed, pressurized dome that sits astride a river. Inside the city, the boys are confronted with the actual aliens, which they refer to as the Masters. Human males are slaves inside the cities, while beautiful females are killed and preserved for the Masters to admire. The Masters live under environmental conditions lethal to unprotected humans; for slaves, who are provided with breathing masks, the artificially increased gravity inside the cities rapidly wastes them away; hence the annual sporting competitions to select the fittest of the human stock to attend the Masters' needs.
While Fritz is severely abused by his Master, Will learns much about the Masters' origins and habits from his Master. Eventually the Master reveals an upcoming operation in which the Earth's atmosphere is to be replaced by the Masters' toxic air, eventually killing off all life on Earth and enabling the Masters to assume full control of the planet. Will meticulously records every piece of information in a diary. One day, the Master finds that diary and confronts Will, who kills him to maintain the secret.
With time running out, Will and Fritz prepare their escape via the river which flows through the city. Will manages to escape, though he nearly suffocates but for the timely assistance of Beanpole, who has been waiting hidden in the ruins surrounding the Tripod colony. The two wait for Fritz, but he does not appear, and in the end the coming winter forces them to return to the White Mountains without him.
The title refers to the gold colour prevalent in the Masters' cities, and that the increased gravity inside the cities makes the human slaves feel as though their bodies have become leaden.
The Pool of Fire (1968) 
Will returns to the headquarters of the Resistance after several months in the City of Gold and Lead, where he and Fritz (who has escaped the city some time after Will) travel to Eastern Europe, the Caucasus region, and the Middle East and set up resistance cells with young boys who question the power of the Tripods. The cult of the Tripods is strong in the Middle East, where the Masters, using the Caps, have replaced Islam with a religion worshipping the Tripods, with some similar features.
The resistance then ambushes a Tripod and captures a Master. Upon the discovery that alcohol has a very strong soporific effect on the Masters without being detectable by them, the Resistance schedules simultaneous commando attacks on the cities. Will is one of the leaders of the attack on the European city.
By introducing alcohol into the city water system, the raiding party is ultimately able to kill all the Masters. The attack on the second city, in eastern Asia, is likewise successful, but the attack on the last city, in Panama, is not. The Resistance attempts an aerial bombing using newly constructed aeroplanes, which also fails due to the Masters' ability to disable the motors. Fritz then leads a successful attack launched from air balloons; however, Henry is killed when he is forced to detonate his bomb by hand, after all the other bombs have been deflected by the city's dome.
The world is liberated from the Masters' thought control and technology is rediscovered rapidly. The Masters' spaceship arrives, launching nuclear devices that destroy the remains of the cities, presumably to prevent the humans from reverse engineering the Masters' technology and launching a retaliatory expedition. Once this happens, the captive Master abruptly dies. Humanity is saved, but the saga ends with a renewal of nationalist sentiments, with tensions building up towards war. The reader is invited by Will's musings to wonder: having mastered the Masters, can people master themselves? Will, Beanpole, and Fritz decide to band together to help unite the people again.
The title of the book refers to the mysterious power source of the Masters' cities, which is a crucial element in the attack on the first city.
When the Tripods Came (1988) 
When the Tripods Came is a prequel written twenty years after the publication of the original "trilogy", and set in the late twentieth century.
In the second book of the main trilogy, one of the Masters tells the main character about the Masters' conquest of the Earth. The plot of the book follows the description of the conquest previously given. It is revealed that the Masters were afraid of the technological potential of humanity and decided on a pre-emptive strike. Unable to defeat humanity in a conventional war, the Masters use their superior mind-control technology to hypnotise part of humanity through a television show called The Trippy Show, and then use the caps to control them permanently when they eventually land. The tripods then cap other people until the capped are in control in most places.
Like the narrator of the original trilogy, the narrator of When the Tripods Came is a young English boy, known as Laurie. As society slowly falls under the control of the Masters, he and his family escape to Switzerland, which has adopted an isolationist stance in order to hold out against the initial invasion. Eventually it is invaded by France and Germany, who have fallen under the subjugation of the Masters, and the narrator is forced to flee into the Alps with his family as the Swiss are also enslaved by the Masters. Here, they establish the "White Mountains" resistance movement that features heavily in the original trilogy, and the book ends on a hopeful note.
Editions have been published by
- Hamish Hamilton (UK First Edition)
- Simon & Schuster (USA First Edition)
- Collier Books
- E. P. Dutton
- Thorndike Press
- Knight Books
- Turtleback Books
- Beaver Books
The series has been translated into Finnish, French, German, Hebrew, Persian, Spanish, and Mandarin Chinese (Taiwan).
Comic books 
Multiple graphic adaptations have been produced, notably including:
- Boys' Life, The Boy Scouts of America magazine, serialized all three books in the trilogy from May 1981 to August 1986. Artist Frank Bolle drew the single page black and white proofs which were then inked by another person.
- In 1985, the BBC initiated BEEB, the BBC Junior Television Magazine, and started to present in picture strip form additional adventures of Will, Henry, and Beanpole on their way to the White Mountains, starting at some unspecified point during the fourth episode of the first BBC serial as the trio pass through ruined Paris, and then heading off at a tangent to the television version. From Issue 6, the boys were accompanied on their journey by a young woman named Fizzio, a character original to the strip. The strips were drawn by John M. Burns and in each issue, they consisted of three pages; the first two in colour and the third in black and white. The storyline was never concluded as BEEB ceased publication after only 20 issues.
Television series 
Series one of The Tripods, broadcast in 1984, which had 13 half-hour episodes written by the well-known author of many radio plays Alick Rowe, covers the first book, The White Mountains; the 12-episode second series (1985) covers The City of Gold and Lead. Although a television script had been written for the third series, it never went into production.
The first series was released on both VHS and DVD. The BBC released Tripods - The Complete Series 1 & 2 on DVD in March 2009.
- "Tripods - The Complete Series 1 & 2 DVD". Amazon.co.uk. Retrieved 2010-06-18.