Seventeen-year-old Miranda lives in future San Diego, a city enclosed in a Dome because chemicals destroyed the natural world. Everything is made of plastic, and wood is more valuable than gold. The daughter of scientists, Miranda is Keeper of The Last Tree, which is dying. She is also great-granddaughter of the famous environmentalist Gabrielle Leigh who foresaw the trees' destruction. By means of The Archives, a multimedia library, Miranda studies Gabrielle's life. Each "visit" through The Portal becomes more real, and Miranda gradually materializes in Twentieth-Century Oregon to explore the Pacific Northwest evergreen forests. She breathes pine-scented air, sees stars and waterfalls, tastes real food like chocolate, and wears cotton clothes. She watches Gabrielle fall in love and is caught in a battle between loggers and "greens." After Gabrielle's husband is killed and their child born in a snow-bound cabin, Miranda tries to stop the other tragedies the Archives foretold. She almost forgets her own world. Back under the future Dome, her family and boyfriend try to keep her from being trapped in the past. Government security forces, afraid that the outside world is too dangerous to explore, hunt them down. Miranda and Gabrielle struggle, between two places and times, to reintroduce trees to the future and lead people outside The Dome.