For the animals in the polar regions, autumn means dramatic battles and epic journeys. Time is running out - the Arctic Ocean is freezing over and the sea ice is advancing at 2.5 miles per day around Antarctica. Polar bears gather in large numbers on the Arctic coast as they wait for the return of the ice. Soon, tempers fray and violent sparring contests break out. Meanwhile 2,000 beluga whales head for one special estuary, a gigantic 'whale spa' where they will thrash their snow-white bodies against the gravel and exfoliate. Inland, the tundra undergoes a dramatic transformation from green to fiery red. Here, musk ox males slam head-first into each other with the force of a 30mph car crash as they struggle to defend their harems. Frisky young caribou males play a game of 'grandma's footsteps' as they try to steal the boss's female. Down in Antarctica, Adelie penguin chicks huddle together in creches. When a parent returns from fishing, it leads its twins on a comical steeplechase - sadly there's only enough for one, so the winner gets the meal. Two months later and the chicks are fully feathered apart from downy Mohican hairdos - they're ready to take their first swim - reluctantly though, as it seems penguins are not born with a love of water! And with good reason - a leopard seal explodes from the sea and pulls one from an ice floe, a hunting manoeuvre that has never been filmed before. As winter approaches and everyone has left, the giant emperor penguin arrives and makes an epic trek inland to breed. The mothers soon return to the sea leaving the fathers to hold the eggs and endure the coldest winter on earth.

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 documentary series reveals the extraordinary riches and wonders of the Polar Regions that have kept people visiting them for thousands of years. Today, their survival relies on a combination of ancient wisdom and cutting-edge science. Most Arctic people live in Siberia, either in cities like Norilsk - the coldest city on earth - or out on the tundra, where tribes like the Dogan survive by herding reindeer, using them to drag their homes behind them. On the coast, traditional people still hunt walrus from open boats - it is dangerous work, but one big walrus will feed a family for weeks. Settlers are drawn to the Arctic by its abundant minerals; the Danish Armed Forces maintain their claim to Greenland's mineral wealth with an epic dog sled patrol, covering 2,000 miles through the winter. Above, the spectacular northern lights can disrupt power supplies so scientists monitor it constantly, firing rockets into it to release a cloud of glowing smoke 100 kilometres high. In contrast, Antarctica is so remote and cold that it was only a century ago that the first people explored the continent. Captain Scott's hut still stands as a memorial to these men. Science is now the only significant human activity allowed; robot submarines are sent deep beneath the ice in search of new life-forms, which may also be found in a labyrinth of ice caves high up on an active volcano. Above, colossal balloons are launched into the purest air on earth to detect cosmic rays. At the South Pole there is a research base designed to withstand the world's most extreme winters. Cut off from the outside world for six months, the base is totally self-sufficient, even boasting a greenhouse.

Sir David Attenborough journeys to both Polar Regions to investigate what rising temperatures will mean for the people and wildlife that live there and for the rest of the planet. David starts out at the North Pole, standing on sea ice several metres thick, but which scientists predict could be Open Ocean within the next few decades. The Arctic has been warming at twice the global average, so David heads out with a Norwegian team to see what this means for polar bears. He comes face-to-face with a tranquilised female, and discovers that mothers and cubs are going hungry as the sea ice on which they hunt disappears. In Canada, Inuit hunters have seen with their own eyes what scientists have seen from space; the Arctic Ocean has lost 30% of its summer ice cover over the last 30 years. For some, the melting sea ice will allow access to trillions of dollars worth of oil, gas and minerals. For the rest of us, it means the planet will get warmer, as sea ice is important to reflect back the sun's energy. Next David travels to see what's happening to the ice on land: in Greenland, we follow intrepid ice scientists as they study giant waterfalls of meltwater, which are accelerating iceberg calving events, and ultimately leading to a rise in global sea level. Temperatures have also risen in the Antarctic - David returns to glaciers photographed by the Shackleton expedition and reveals a dramatic retreat over the past century. It's not just the ice that is changing - ice-loving adelie penguins are disappearing, and more temperate gentoo penguins are moving in. Finally, we see the first ever images of the largest recent natural event on our planet - the break up of the Wilkins Ice Shelf, an ice sheet the size of Jamaica, which shattered into hundreds of icebergs in 2009.

In a late Jurassic forest in what is now China, an Epidexipteryx escapes from a juvenile Sinraptor by climbing a tree. It finds a beetle grub in the tree bark, being shown to use its elongated fingers in a similar way to a modern day aye-aye. However, its prey is stolen by another, larger Epidexipteryx, and after a brief bout of posturing, the smaller individual goes to find more food. It drops a second grub to the forest floor, where the other Epidexipteryx retrieves it, only to be killed by the juvenile Sinraptor. The episode then cuts to a desert in late Cretaceous Mongolia, where a Saurornithoides is shown brooding a nest of eggs. When it leaves the nest, an Oviraptor raids it, fleeing when the troodontid returns. The Saurornithoides is suddenly attacked and eaten by a Gigantoraptor, which then heads to compete in a breeding ritual for mates. The males use their feathers for display, a brief fight between two erupting at one point, allowing the females to choose the best suitor. The episode finally cuts to an early Cretaceous forest in China, where a Xianglong is being hunted by a Microraptor, which uses its feathers to pursue the gliding lizard in the air. A Sinornithosaurus attacks it, and after a brief chase the Microraptor manages a lucky escape. The Sinornithosaurus, alongside two other members of its species is then shown hunting a Jeholosaurus and its three young. The group brings down the parent, the narrator explaining that their possibly venomous bite allowed them to tackle animals much larger than themselves. A montage is then shown of the feathered dinosaurs featured in the programme, with the narrator saying that Microraptor not only hints at how flight might have developed, but also that dinosaurs still live amongst us today, as birds.

In late Cretaceous Canada, in what will be known as Dinosaur Provincial Park, a Daspletosaurus stalks a Chasmosaurus in a forest, but loses the element of surprise and is forced to retreat. The Chasmosaurus comes across a younger Daspletosaurus, before being ambushed by a group of the tyrannosaurids. The episode then cuts to the high Arctic, where Edmontosaurus are hunted by a large species of Troodon. The theropods attack at night, separating a juvenile from the herd and severely wounding it, only to be driven away by an adult. In the morning, they return to eat the carcass of the juvenile, which died during the night. The episode returns to the Daspletosaurus, who chase and bring down the Chasmosaurus. The larger adults bully the youngsters off the carcass, forcing them to wait until they have finished. The episode then cuts to Madagascar, where a mother Majungasaurus (an Abelisaurid) and her two offspring chase a group of Rahonavis off a carcass. However, they are temporarily driven off themselves by a male Majungasaurus. But, after he steals some food from one of the young, the female attacks him, before she and her young cannibalise his body. The episode returns once again to North America, where the Daspletosaurus are waiting for the annual migration of Centrosaurus. They attack during a rainstorm, killing some of the ceratopsians. The Centrosaurus make it to a flooded river and begin to swim across, and although many make it to the other side, some are caught by giant crocodilians or are severely wounded by floating debris and thus drown, or drown for unseen reasons. In the morning, the carcasses attract scavengers, including the Daspletosaurus. A montage is then shown of Daspletosaurus and Majungasaurus, the narrator saying that together, the tyrannosaurids and abelisaurids were the last of the killer dinosaurs.