The principal character in The Map That Changed the World is William Smith, a self-educated geologist who is considered a pioneer in the field. He is remembered both for his work in development of a key idea in geology and his colorful maps. William was born in 1769 in Oxfordshire, and, according to the book, he was quickly preoccupied with things underground. He also had above-ground interests, however, which helped him in his early journey toward an understanding of how the Earth recorded its history and how different parts of the world changed over time.

Smith's uncle was a baker, and he would watch with interest as he weighed out his wares using a pound-stone. The stone attracted Smith's interest because it was unlike any of the other stones in the area. Eventually, he was able to identify it as a petrified sea urchin. But how did a remnant from the sea make it all the way to Oxfordshire? The question loomed in his mind.

During one of his underground forays, Smith was in a mine shaft in Somerset when he noticed that there were layers of matter embedded in the walls. What if, Smith asked himself, those layers were evidence of sediment that had accumulated on the Earth over time, with the oldest being the furthest down and the newest being closer to the surface? He would eventually find material in very old layers that suggested that at one time the sea was in Oxfordshire, thus the presence of the sea urchin. He had discovered strata, the important idea of layers of sediment that told the story of the Earth. In recording them, he also developed brilliant and colorful maps of his beloved country.

As he was perfecting those maps, another character appears in The Map That Changed the World: George Greenough, the president of the Geological Society of London, which rejected Smith as a geologist but stole information from his maps to create a map of Britain. Smith ended up a debtor at a time when one could be jailed for debt as a criminal offense, so he also served prison time.

The story of Smith is a story of class prejudice and triumph over obstacles. He is remembered as a pioneer of geology and his enemies as a mean footnote to his accomplishments. His discovery of strata is a universal testament to his ingenuity and passion for the underground world.