As we follow the grip of winter over the course of six freezing months, we chart the fortunes of Yellowstone's wildlife in a finely balanced fight to survive. Bison and elk struggle to feed with the grass buried under some of the deepest snow in America, while the wolf grows stronger during the harsh winter months. All of their fates are ultimately linked to the power of Yellowstone's volcanic heart.

As the spring melts the winter snow, the full extent of Yellowstone is gradually revealed. Now, from the surrounding lowlands herds of elk, pronghorn and bison return from their winter feeding grounds to take advantage of America's richest natural grasslands. Grizzly bears emerge from hibernation to teach their young to hunt fish and how to survive in the Alpine peaks. But just as it seems the easy living of summer has arrived, a new danger emerges.