It is the greatest technological challenge ever undertaken by humankind. The quest to produce the ultimate energy solution, Fusion, a reliable source of clean power. What if we could create here on earth, our very own version of a star, a machine that makes power from nuclear fuel? But so immense are the challenges, the goal of a power-producing fusion reactor has so far remained elusive and out of reach... until now.

On the second part of his journey through the dark ages Richard Rudgley continues into the age of the wandering peoples, the Volkerwanderung. These Northern people enjoyed a golden age unaffected by Rome and just 30 years after the Romans relinquished Britain, the 'Anglo-Saxons' made their move. The bedraggled legions are in retreat. Walls are pulled down. Mosaics shattered. And yet there never was a people called Anglo-Saxon. We look at the lasting influence of Saxon leaders like Alfred the Great, and his blue print for social justice.

The great mountain ranges are some of the planet's most spectacular landscapes, but they are unforgiving places to live in, and only a few animals have what it takes to live at extreme altitude. Mountain animals are amongst the most elusive in the world, and this film provides unique and intimate glimpses into their secretive lives. Witness the moment four snow leopards come together when a mother and cub become trapped between two rival males. Join grizzly bears as they dance against trees to rub off their winter fur and soar with golden eagles hunting amongst Europe's snow-capped peaks.

Jungles provide the richest habitats on the planet - mysterious worlds of high drama where extraordinary animals attempt to survive in the most competitive place on earth. Flooded forests are home to caiman-hunting jaguars and strange dolphins that swim amongst the tree tops, while in the dense underworld, ninja frogs fight off wasps and flying dragons soar between trees. Acrobatic indri leap through the forests of Madagascar, while the jungle night conceals strange fungi and glow-in-the-dark creatures never filmed before.

The world's deserts are lands of extremes that force animals to come up with ingenious ways of coping with hostile conditions, giving rise to the most incredible survival stories on earth. A pride of desert lions are so hungry they risk hunting a giraffe several times their size, while male sandgrouse fly 120 miles each day to the nearest waterhole and dice with death to collect water for their chicks. Filmed for the first time, a tiny bat does battle with one of the world's deadliest scorpions, and in Madagascar, a locust swarm of biblical proportions is seen as never before.

Grasslands cover one quarter of all land and support the vast gatherings of wildlife, but to survive here animals must endure the most hostile seasonal changes on the planet. From Asia's bizarre-looking Saiga antelope to the giant anteaters of Brazil, grassland animals have adapted in extraordinary ways to cope with these extremes. In the flooded Okavango, lions take on formidable buffalo in epic battles, on the savannah bee-eaters take advantage of elephants to help catch insects and, on the freezing northern tundra, caribou embark on great migrations shadowed by hungry Arctic wolves.