In one of the earth's most amazing migrations, more than 100,000 caribou trek thousands of miles each year over high mountain ranges, through snowy passes, and across icy rivers. But they have to battle more than just the brutal elements. Hungry wolves, huge grizzly bears, human hunters, and hordes of bloodthirsty insects besiege the herd as it travels to its one safe haven—Alaska's Arctic National Wildlife Refuge. There, those that survive the trip have a few peaceful weeks to give birth and prepare their calves for the harsh year ahead.

Karsten Heuer and his wife, Leanne Allison, are the only humans ever to become part of a caribou herd and join it on its arduous journey. They shared the same mind-numbing cold, the endless miles of physical hardship, and all the dangers along the route to chronicle the epic battle for survival these animals face. To keep up, they had to move, act, and even think like caribou. Karsten and Leanne's incredible adventure gives us a window into a world that we have never seen before.