In the fourth Book of Ember, The Diamond of Darkhold, Lina and Doon are curious when a roamer comes into town with a mysterious book. They trade a match for the book and discover it only contains eight pages. This is because the same person used the rest of the paper to create fires. On the front of the book, printed in gold letters, it says, "For the People of Ember." Since they cannot make any sense of the book, they decide to go back to Ember. But when they return, the family of squatters have taken over the city. The Troggs (Washton, Kanza, Minny, Yorick and an adopted boy called Scawgo) believing that they own Ember, rename it Darkhold. They capture Doon and Lina goes back to Sparks for help. Meanwhile, Lizzie (from Ember), Kenny, and Torren (both from Sparks) realize that the two are gone, and decide to go and try to find them. They don't succeed and a search party goes to look for them. While Doon is with the family they show him a diamond they found just outside Ember. Doon steals the diamond and escapes from Ember with help from Scawgo. While escaping, he also breaks the pipe connecting the generator to a waterwheel that created power for Ember, therefore shutting down Ember's lights for good. He finds Lina, who treter. A weary group of roamers, which are actually the Troggs, come into town one day, and a diamond is returned to them. But soon they learn the truth. There are hundreds of diamonds, thousands, and they can power all sorts of electronic devices. They also learn that Doon and many others lived in Ember and that they left and evacuated because Ember was dying. So the Troggs come to live in Sparks as well. And in the end, it is revealed that in the future, cities are rebuilt with the power of the diamonds.
The story begins with the Builders discussing what the Emberites should do when they are released from the city. It is decided that a recent invention (which is later shown to be a diamond-like gadget) will be stored alongside a guide providing information regarding its use. However, these items are left undiscovered when the Emberites escape.
Nine months after Ember and Sparks have reached a truce, a roamer comes into town with a mysterious book, on the front of which is printed "For the People of Ember". Lina Mayfleet and Doon Harrow obtain the book via barter but soon learn it contains only eight pages, as the trader used the rest to light campfires. Unable to make sense of the book's remaining contents, Lina and Doon decide to return to Ember to investigate.
When they arrive, they discover that a family has taken over the darkened city. The Troggs — Washton, Kanza, Minny, Yorick and an adopted boy, Tim, whom they have dubbed "Scawgo" — believe they own Ember and have renamed it Darkhold. They capture Doon, but he manages to communicate with Lina and she goes back to Sparks for help. Meanwhile, Lizzie Bisco (from Ember), Torren Crane and Kenny Parton (both from Sparks) attempt to locate Lina and Doon but don't succeed, prompting another, larger search party to go looking for them. While Doon is with the Troggs, they show him a diamond they found just outside Ember, but Doon steals the diamond and escapes. In the process, he also breaks the pipe connecting the generator to the waterwheel that created power for Ember, thereby dimming Ember's lights for good.
When Doon finds Lina, a pack of hungry wolves is threatening her. Doon throws the diamond at the wolves to frighten them away, but shatters it and twists his ankle in the process. Lina treats his wound and takes him to the place where the book and the diamond were discovered. There, they uncover shelves filled with hundreds upon hundreds of diamonds, and Lina and Doon realize that the devices are solar-powered sources of electricity. Ultimately, the people of Sparks, as well as the former Emberites, help them retrieve the diamonds and other items left behind in Ember, enough to ensure both groups will survive the coming winter. Before returning to the Earth's surface, Lina collects the drawings of her "dream city" that she sketched at her former home, then goes to City Hall and stands on top of the building to say "Goodbye, Ember — forever."
Later, the Troggs arrive in Sparks and decide to settle there after learning why Ember/Darkhold was abandoned. Lizzie takes an interest in Tim (abandoning her earlier hope of becoming Doon's girlfriend), while Lina gets a horse named Fleet and becomes a messenger as she was in Ember. The last chapter reveals that Lina and Doon eventually fall in love and have children together. Many years later, after new cities have been built using solar power, one of their descendants views the now-fragile pictures drawn by Lina as a girl in Ember and notes how strongly they resemble a world she didn't live to see but nonetheless helped create.