In the far north of our planet, lie vast frozen lands of snow-covered forests and open tundra.
During the months to come, they will be transformed by the greatest seasonal changes on earth.
But now, in midwinter, conditions are so hostile that life seems all but impossible.
But... not so. In this world of extremes, animals must go to extraordinary lengths to survive.
February, northern Canada. The harsh winter has brought together an extended family of wolves.
But this is no ordinary pack. There are 25 of them.
A super-pack, one of the largest ever recorded.
They're ravenously hungry. They may have walked over 150 miles since they last had a meal.
But now, at last, the lead male and female have picked up a trail.
It's been left by the only substantial prey there is here at this time of year... Bison.
They're formidable animals, not easy to attack.
The bulls are ten times the size of a wolf.
And they have horns with which they could rip any attacker to pieces.
Gathered together and on guard, they appear invulnerable.
The wolf pack's only chance... is to try and break up the group and isolate one of them.
To do that, they will have to work as a team and try to start a stampede.