The story of The Tripods is post-apocalyptic. Humanity has been conquered and enslaved by "the tripods", unseen alien entities who travel about in gigantic three-legged walking machines (the unsophisticated humans believe the walking machines themselves to be their living overlords). 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 previous ages are still used, giving individuals and homes a rather anachronistic appearance. Humans are controlled from the age of 14 by cranial implants called "caps", which suppress curiosity and creativity and leave the recipient placid and docile, incapable of dissent. The caps cause them to adore the tripods as their saviours. Some people whose minds are crushed under the pressure of the cap's hypnotic power become vagrants who wander the countryside shouting nonsense.
Disney has owned the rights to The Tripods since 1997. It has been reported that a cinematic version is in pre-production with Australian-born director Gregor Jordan signed on to rewrite and direct for Walt Disney's Touchstone Pictures label.
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. We learn 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. As society slowly falls under the control of the Masters, he and his family escape to Switzerland, which adopted an isolationist stance 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.
The Tripods trilogy can be read and understood without any need to have read the prequel.
While written for a young audience — being rather short (under two hundred pages) and having an unsophisticated vocabulary — the book is swiftly plotted and filled with narrow escapes, except for a brief period during which the boys live at a manor owned by a wealthy French count. Will forms a strong relationship with his preteen daughter, Eloïse, and is heartbroken when Eloïse is chosen "queen of the tournament" at an athletic competition that the count hosts for knights living in the surrounding countryside, for Eloïse must then go off to serve the Tripods in their domed city (the Tripods, it turns out, actually have three cities: one in Germany, one on the Chinese coast, and one on the Panama Canal).
It's been a year since Will, Henry, and Beanpole arrived at the White Mountains. The Resistance now charges Will, Beanpole and a young German boy, Fritz, now wearing realistic yet harmless caps, to infiltrate a Tripod city by competing in a sporting exhibition called the Games (very similar in nature to the Summer Olympics) in which the winners of the events are to be offered to the Tripods for service. Will, a boxer, and Fritz, a runner, win their respective contests, while Beanpole is unsuccessful in the jumping events. Will, Fritz, and the other 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 (apparently a tributary of the river Rhine) somewhere in northwestern Europe. Inside the city, the boys are confronted with the actual Aliens, which call themselves the Skloodzi (though humans refer to them as the Masters). They are three-legged, three-eyed, three-tentacled creatures from a planet with a stronger gravitational field and a hotter ambient temperature, and which breathe a greenish gas toxic to humans--conditions on the Masters' world are therefore re-created inside the City. The humans in the City are treated as slaves and pets — being given their own airlocked rooms in which they can live (though not very comfortably) without gas masks when their Masters do not require their services.
Since the Masters are unaware that Will and Fritz are false-capped, they don't consider the possibility of hostile behaviour. Thus, the boys are able to spy unobtrusively on a significant portion of the city. In addition, Will's Master develops a special fondness for him that causes the Master to occasionally reveal details of the Masters and their civilization. Will thereby learns that the stakes of the mission are much higher than anyone had foreseen: the Masters have initiated a project to replace the Earth's atmosphere with their own in preparation for the colonization of the planet, which will obliterate humanity and much other Earth life in the process. The spaceship which carries the processing equipment is already on its way, and is due in four years.
Will compiles a journal of his observations and findings for the benefit of the resistance group. His Master ultimately discovers the journal, and in desperation, Will attacks and kills his Master, accelerating the boys' escape plans. Will and Fritz find an opening in the golden wall through which water runs. Will escapes but Fritz stays, arguing that it would look suspicious if two slaves suddenly went missing. Will is saved by Beanpole, who was living outside the city waiting for his friends' escape. Will and Beanpole wait for Fritz to come out of the city, but many days of vain waiting and the coming of winter cause them to head back to the White Mountains.
The title of the book refers to the great gold coloured wall surrounding the Masters' city, and to Will's comment that increased gravity inside the city made him feel as though his body had become as heavy as lead.
By introducing alcohol into the city water system, the raiding party is able to incapacitate all of the Masters and ultimately to destroy the integrity of the city's sealed environment, killing 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. After aerial bombing attempts from newly constructed airplanes fail — because the Masters can disable the motors from a distance, presumably with an EMP — the third city is eventually destroyed in an attack launched from air balloons, albeit only after Henry resorts to suicide bombing.
The world is liberated from the Masters' thought control and technology is rediscovered rapidly. The Masters' spaceship finally arrives, only to launch nuclear devices that destroy the remains of the cities, presumably to prevent the humans from reverse engineering the Masters' technology and use it to launch a retaliatory expedition against them. Humanity is saved, but the saga ends with a renewal of nationalist sentiments. 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.
Multiple graphic adaptations have been produced, notably including:
Season one of the Tripods, broadcast in 1984, which had 13 half-hour episodes, covers the first book, The White Mountains; the 12-episode second season (1985) covers The City of Gold and Lead. Although a television script had been written for the third season, due to a campaign against TV Sci-Fi by Michael Grade, the Controller of BBC at the time, it never went into production.
The first season is available on DVD but the second has yet to be released. Although preorders were taken in 2003 for the second season, they were all cancelled. There was another release date for Tripods - The Complete Series 1 & 2 on DVD for September 2007, but this has changed to January 2009.
It can be noted that the series introduced several minor changes from the book, notably the shape of the Masters and Tripods, which have tentacles (although the Tripods do have a mechanical claw-arm that they sometimes use); in the book, gravity inside the Golden City was increased artificially, which is not mentioned in the TV series; the introduction of "cognoscs", spiritual life-forms vastly superior to the Masters themselves; and more interesting main characters, including love interests for both Will and Beanpole. The original texts have almost no female characters at all. Youd was recently asked about this for an interview on Wordcandy, replying that at the time of writing the series, it was generally accepted that girls would read books with boy main characters, but not vice versa. He also stated that he felt the addition of an entire family of girls to the TV series was somewhat "over the top". The series is also notable for featuring non-humanoid aliens, which was uncommon at the time. There is also one major change in the series. In the series people are capped at 16, however in the books they are capped at 12-13. This was a change made by the BBC Producers, since hiring child actors would have been more expensive since additional crew would have to be hired (such as teachers, additional medical staff, specialised stunt persons, and chaperones) and the production time would have to be extended due to the actors having to attend on set classes, and child actors by law had to have the hours they worked on set reduced, thus increasing the production costs greatly.
BBC Enterprises licensed a video game adaptation of the TV series in 1985. It was designed by Watermill Productions and published by Red Shift.
References to both the books and the artwork from them can be found on the 2006 self-titled album by Gatsbys American Dream. The artwork features a tripod seemingly about to crash into a train, and there are songs called "The White Mountains" (Book 1, Chapter 10) and "My Name Is Ozymandias" (Chapter 2). There is also an association with the story of "War of the Worlds", with an interpretation of the The Tripods being a 'what if?' response to idea of the tri-phile alien invaders not being killed off by earthly bacteria.