The Tripods Trilogy delves into the importance of freedom of thought. In The White Mountains and its sequels, a small group of free people battles against alien invaders known as the Masters, who dominate humanity by "Capping" children around the age of fourteen. After being Capped, humans can no longer resist the Masters' commands. They lose not only their autonomy but also the knowledge of their own history, causing Earth to regress to a primitive technological state.

Though the Masters seem incredibly powerful, they cannot breathe Earth's atmosphere naturally. Consequently, they must stay within their domed cities, and if they wish to travel, they must use the mysterious Tripods—large metal machines that walk on three legs. The Masters' greatest vulnerability lies in their belief that humans lack the will and capability to fight for their freedom. The Tripods Trilogy illustrates that the quest for freedom can be both painful and expensive, and maintaining newly gained freedom can be the toughest challenge. The central character in The White Mountains, Will Parker, is a determined and independent thirteen-year-old. Although Will is sensitive and thoughtful, he can also be reckless, somewhat dishonest, prone to jealousy, and a poor judge of character. Throughout his adventures, Will learns valuable lessons about friendship, cooperation, and self-discipline. His companions—his practical but unimaginative cousin Henry and an intelligent French boy named Jean-Paul Deliet, nicknamed Beanpole—are not as intricately developed as characters. Over time, Henry, once an old rival, becomes a close friend, and Beanpole's scientific mindset proves invaluable to the group.

At the beginning of the novel, a mysterious wanderer named Ozymandias visits the village of Wherton. He pretends to be insane, speaking in what sounds like nonsense when adults are present. However, in private, he explains to Will the true cost of Capping: specific commands are implanted in the mind, subjugating the individual to a small, "inescapable and irresistible" voice of authority. Ozymandias eventually provides Will with a brief history lesson, summarizing world events since the Masters' arrival—a span of about one hundred years that "to the Capped . . . is the same as ten thousand."

Will and Henry escape their home to avoid Capping and, together with Beanpole, journey across Europe toward the "White Mountains." At one point, when Will becomes ill, the boys are taken in by an aristocratic French family. The world of the Chateau de la Tour Rouge is elegant, with feudal customs and traditions. Will falls in love with the family's daughter, who returns his feelings, tempting him to remain in this peaceful, luxurious haven instead of continuing the fight against the Tripods. However, he is horrified to discover that Eloise, the girl he loves, has already been Capped and is eagerly preparing to serve in one of the Masters' grand domed cities.

The magnificent domed cities stood before us. The key difference was that we weren't strangers—we were outlaws.
Before the boys arrive at the Alpine headquarters of the freedom fighters, they encounter numerous perilous challenges. At one point, Will is captured by a Tripod and has a tracking device implanted under his arm. Beanpole discovers the device and performs a painful operation on Will to rid him of its influence. Later, similar to knights demonstrating their bravery by slaying a dragon, the boys confront and destroy a Tripod using an old but still effective hand grenade.