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.

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.

Earth may seem like the most hospitable planet in the solar system. But look again. Startling new discoveries reveal the blue planet has been plagued by more chaos and destruction than scientists once imagined. Stand on the Earth billions of years ago as a primitive planet called Theia slams into it. Shiver as our entire globe is frozen over like a gigantic snowball. Feel the heat as mammoth volcanoes scorch the landscape and darken the sky. From a cosmic gamma ray burst frying away the ozone layer to an Everest-size asteroid slamming into the ocean, we'll reveal new information about how these unparalleled events drove life to the brink of total extinction. Out of this continuous devastation, how has our planet--and life--got to where it is today? Are the worst days behind us--or lurking in the distant future?

In the high-stakes game of big-wall climbing, the Shark’s Fin on Mount Meru may be the ultimate prize. Sitting at the headwaters of the sacred Ganges River in Northern India, the Shark’s Fin has seen more failed attempts by elite climbing teams over the past 30 years than any other ascent in the Himalayas. To undertake Meru, says Jon Krakauer, the bestselling author of Into Thin Air, "You can’t just be a good ice climber. You can’t just be good at altitude. You can’t just be a good rock climber. It’s defeated so many good climbers and maybe will defeat everybody for all time. Meru isn’t Everest. On Everest you can hire Sherpas to take most of the risks. This is a whole different kind of climbing."

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.

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.