In the last episode, Iain discovers the remarkable impact of just one plant: grass. On the savannah of South Africa he sees how grass unleashed a firestorm to fight its greatest enemy, the forests. He shows how cutting your finger on a blade of grass shows us how it transformed life in the oceans. In Senegal, he meets the cleverest chimps in the world. And, in the ruins of the oldest temple on Earth, he tells the extraordinary story of how grass triggered human civilisation.

Between the 1920s and the 1960s the world's great powers sent vast military-style expeditions to conquer the peaks of the Himalayas, with Everest at their head. This was a great game played - camera in hand - by Imperial Britain, Nazi Germany and superpower America. As a result, Himalayan mountaineering's most iconic, epic and tragic moments didn't just go down in history, but were caught on film - from the deaths of Mallory and Irvine on Everest in 1924, to Everest's final conquest in 1953 by Hillary and Tensing. Using footage never before seen on British television, this is the story how of how film-makers turned the great peaks into great propaganda.

Leaving Everest base camp to take the high road to Lhasa to see what the Chinese have done to Tibet. He sees that religion is once again tolerated, while the old Tibetan centre of the city is being replaced with modern Chinese shopping malls and nightclubs. Following the pilgrims to the holy Namtso Lake, he gets warm in a hot spring before learning how to milk a yak with a nomad family.

During a Gurkha recruitment Palin is disturbed by Maoist insurgents, but survives to suffer as he climbs to 15,000 feet and sees the majesty of Annapurna Sanctuary. In Kathmandu he is blessed by the Nepalese king before meeting some holy men. Crossing into Tibet, he meets his first yaks at the highest monastery in the world, before heading up the Rongbuk glacier towards the summit of Everest.

Do other worlds tremble and heave with geological life? On Mars, we found a volcano to dwarf Mount Everest. On the moons of the giants our understanding of volcanoes was changed forever.

Each night, across the planet, our cities illuminate the darkness. Their lights burning brighter than ever before. And whilst we're asleep, animals are making the most of this new neon landscape. The sunset in Los Angeles, Chicago, and Toronto creates worlds of opportunities for the cleverest animals willing to take the risk.