With the brutal extremes of winter looming, naturalist Casey Anderson says good-bye to his grizzly bear companion Brutus, and treks deep into Yellowstone's backcountry to experience the park like most visitors never will. Casey braves severe cold, snowstorms and dangerous animal encounters as he explores how the park's diverse wildlife prepares for a long, harsh winter. On his more than 40-mile journey, Casey tracks the notoriously elusive mountain lion and follows wild grizzlies as they hunt for food to carry them through hibernation. Casey also heads to a cutting-edge research facility where scientists are gaining new insight into the biology behind animals' seasonal changes. Here, Casey wakes a hibernating grizzly bear, holds a bear's brain in his hands, and tests his balance in a contest with bighorn sheep.