It is high summer in the polar regions, and the sun never sets. Vast hordes of summer visitors cram a lifetime of drama into one long, magical day. They must feed, fight and rear their young in this brief window of plenty.

Summer is a tough time for the polar bear family, as their ice world melts away and the cubs take their first swimming lesson. Some bears save energy by dozing on icy sunbeds, while others go egg-collecting in an arctic tern colony, braving bombardment by sharp beaks.

There are even bigger battles on the tundra - a herd of musk oxen gallop to the rescue as a calf is caught in a life and death struggle with a pair of arctic wolves. But summer also brings surprises, as a huge colony of 400,000 king penguins cope with an unlikely problem - heat. The adults go surfing, while the woolly-coated chicks take a cooling mud bath.

Nearby, a bull fur seal is prepared to fight to the death with a rival. Fur flies as the little pups struggle desperately to keep out of the way of the duelling giants.

Further south, a minke whale is hunted amongst the ice floes by a family of killer whales. The dramatic chase lasts over two hours and has never been filmed before. The killers harry the minke whale, taking it in turns to wear it down. Eventually it succumbs to the relentless battering. Finally, comical adelie penguins waddle back to their 500,000-strong colony like clockwork toys. The fluffy chicks need constant feeding and protection as piratical skuas patrol the skies. When an unguarded chick is snatched, a dramatic dogfight ensues.

There is no greater test for life than winter, as temperatures plummet to 70 below and winds reach 200kph. Darkness and ice extend across the polar regions and only a few remarkable survivors gamble on remaining.

We join a female polar bear trekking into the Arctic mountains to give birth as the first blizzards arrive. Out on the frozen ocean, the entire world's population of spectacled eider ducks brave the winter in a giant ice hole kept open by ferocious currents. Arctic forests transform into a wonderland of frost and snow - the scene of a desperate and bloody battle between wolf and bison, but also where a remarkable alliance between raven and wolverine is made. Beneath the snow lies a magical world of winter survivors. Here tiny voles dodge the clutches of the great grey owl, but cannot escape the ultimate under-show predator - the least weasel.

Midwinter, and a male polar bear wanders alone across the dark, empty icescape. Below the snow, polar bear cubs begin life in an icy den while fantastical auroras light the night skies above. In Antarctica, we join male emperor penguins in their darkest hour, battling to protect precious eggs from fierce polar storms. Weddell seals escape to a hidden world of jewel-coloured corals and alien-looking creatures, but frozen devastation follows as sinister ice stalactites reach down with deadly effect.

The sun finally returns, and with it comes the female emperor penguins, sleek and fat, ready to deliver the first meal to their precious chick. Having survived winter, this ultimate ice family now have a head start in raising baby. The adelies flood back and as the ice edge bustles with life, male emperor penguins can finally return to the sea.