From lush cloud forests to bare summits that take your breath away, the higher you climb the tougher life gets on a mountain. Human Planet explores the extraordinary ways in which people survive at extreme altitudes where nature becomes utterly unforgiving. In the Altai Mountains in Western Mongolia the vast open spaces make hunting for animals almost impossible, so the locals have forged an astonishing partnership with golden eagles which can do the hunting for them. On the precipitous cliffs of the Simien Mountains of Ethiopia we join a young boy locked in a dramatic battle with fearsome gelada monkeys which are hell-bent on raiding his family's meagre grain harvest. In the Himalayan state of Nepal - the roof of the world - we witness a rarely seen ceremony: a sky burial. In a land where there is little wood to burn for cremation, and where burying the dead is virtually impossible, the dead are fed to vultures in the ultimate reverence of nature.

We face one of the greatest challenges in the history of humanity, to eliminate the fuels that have driven progress and technology for over a century, while our thirst for energy only grows. The sun is the biggest source of energy in the solar system. It's like a nuclear reactor in the sky, and it provides endless power. If we can harvest just fractions of this, it can power all our consumption. Innovators are searching for new ways to capture more of the sun’s power, and make it available through the night, everywhere.

Stone opens the episode on Vietnam with a review of American intervention in Latin America. Lyndon Johnson, according to Stone, disregarded JFK’s “memo” about withdrawing troops and instead escalated. The Cataclysm in Vietnam as the war reaches a turning point - there's no going back. The betrayal by Richard Nixon.

Mountain biking requires endurance, core strength and balance, bike handling skills, and self-reliance. Advanced riders pursue both steep technical descents and high incline climbs. In the case of freeriding, down hilling, and dirt jumping, aerial manoeuvres are performed off both natural features and specially constructed jumps and ramps. Follows several individuals breaking out of their mundane lives and entering into the adventurous thrill-seeking world of this sport.

Mark has never had a girlfriend, but he wants love more than anything. Maddi and Mark are going on their second date at the Australian Museum. Olivia has had 'a bazillion' unrequited loves but is not ready to give up. Maddi dates a new man who impresses her with his musical talent.

According to the laws of physics we can never travel faster than the speed of light...or can we? Learn all about light speed, the ultimate constant in the universe.