Episode 1: In 1851, a band of Indian fighters in California encounters a place of astonishing beauty, setting in motion events that bring other newcomers to Yosemite Valley: artists, writers, entrepreneurs, tourists, and eventually John Muir, who becomes a national voice for preservation. Meanwhile, reports emerge from Wyoming Territory of a fantastical place at the headwaters of the Yellowstone River. An exploration confirms the rumors, and in 1872 Congress creates the world's first national park at Yellowstone, but does nothing to provide for its protection. In 1886, General Phil Sheridan and the U.S. Cavalry ride to the park's rescue.

Episode 2: At the end of the 19th century, some Americans begin to question the nation's headlong rush across the continent that has devastated forests and ravaged entire species of animals. Conservation's greatest champion is the new president, Theodore Roosevelt, who creates parks and wildlife refuges, and then audaciously uses the Antiquities Act to set aside 800,000 acres of the Grand Canyon as a national monument. John Muir fights the battle of his life to prevent the city of San Francisco from burying the Hetch Hetchy Valley in Yosemite National Park under a reservoir, and dies broken-hearted after he loses.

Episode 3: America boasts a dozen national parks as the park idea turns 50 years old. A millionaire businessman named Stephen Mather impulsively accepts the offer to oversee them for one year. Mather and his right-hand-man Horace Albright launch a campaign to publicize the parks as a unified system and to persuade Congress to create a single agency to oversee it: the National Park Service, established in 1916. Mount McKinley, Zion, Bryce Canyon, Arches, Acadia and Hawaii's volcanoes are set aside as national parks, but Mather's top priority is in Arizona. After a bitter fight, the Grand Canyon is designated a national park in 1919.

Episode 4: As the nation enters the 1920s, Stephen Mather and Horace Albright ally themselves with the automobile to "democratize" the national parks and attract more Americans to them. Nebraskans Margaret and Edward Gehrke begin collecting parks each summer, while Glenn and Bessie Hyde spend their honeymoon in a homemade boat on the raging Colorado River through the Grand Canyon. Horace Kephart, a reclusive writer, and George Masa, a Japanese immigrant and photographer, launch a campaign to save the virgin forests of the Smoky Mountains from destruction by making it a national park.

Episode 5: A new president, Franklin D. Roosevelt, expands the national park idea to embrace battlefields and other historic and iconic sites. He enters pitched battles to create national parks on the Olympic Peninsula, Florida's Everglades, Wyoming's Teton Mountains, and California's High Sierra; he also creates the Civilian Conservation Corps to provide young men with jobs improving conditions at national parks. George Melendez Wright, a young Park Service employee, begins arguing that the parks are not doing enough to protect wildlife in their natural state. In Seattle, Iwao and Hanaye Matsushita fall in love with Mount Rainier National Park; and in California, another Japanese immigrant, Chiura Obata, finds inspiration for his art in Yosemite. When they are interned during World War II, they all find solace in their memories of the national parks of their adopted country.

Episode 6: After World War II, an increasingly mobile and affluent nation begins placing demands on the parks as never before, and the parks are in danger of being "loved to death." A Park Service biologist named Adolph Murie argues that ingrained practices such as killing predators runs counter to the purpose of national parks, while David Brower of the Sierra Club mobilizes public opinion to defeat Congressional proposals for dams in pristine places. In the 1970s, when President Jimmy Carter uses the Antiquities Act to set aside 56 million acres in Alaska, a huge uproar results - and the largest grassroots movement in conservation history fights for the creation of seven new Alaska parks, adding 47 million acres, more than doubling the size of the park system.