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This episode explores the nature of the greenhouse effect (discovered by Joseph Fourier and Svante Arrhenius), and the evidence demonstrating the existence of global warming from humanity's influence. Tyson begins by describing the long-term history of the planet Venus; based on readings from the Venera series of probes to the planet, the planet had once had an ocean and an atmosphere, but due to the release of carbon dioxide from volcanic eruptions, the runaway greenhouse effect on Venus caused the surface temperatures to increase and boiled away the oceans. Tyson then notes the delicate nature of the amount of carbon dioxide in the atmosphere can influence Earth's climate due to the greenhouse effect, and that levels of carbon dioxide have been increasing since the start of the 20th century. Evidence has shown this to be from mankind's consumption of oil, coal, and gas instead of from volcanic eruptions due to the isotopic signature of the carbon dioxide. The increase in carbon dioxide has led to an increase in temperatures, in turn leading to positive feedback loops of the melting polar ice caps and dethawing of the permafrost to increase carbon dioxide levels. Tyson then notes that humans have discovered means of harvesting solar power, such as Augustin Mouchot's solar-driven motor in the 19th century, and Frank Shuman's solar-based steam generator in the 1910's. Tyson points out that in both cases, the economics and ease of using cheap coal and oil caused these inventions to be overlooked at the time. Today, solar and wind-power systems would be able to collect enough solar energy from the sun easily. Tyson then compares the motivation for switching to these cleaner forms of energy to the efforts of the Space race and emphasizes that it is not too late for humanity to correct its course.

Tony Accardo have taken the reins of the criminal empire built by their mentor Al Capone. Thanks to Sam Giancana's takeover of the policy racket, the gangsters are now making millions. With so much money coming in, Accardo knows they need a place to hide it. They find their answer in a growing city where gambling is now legal: Las Vegas. However, the New York mafia is already there, led by criminal mastermind Meyer Lansky.

In 1985, two adventurous young mountaineers, Joe Simpson and Simon Yates, set off to climb the treacherous west face of the Siula Grande in the Peruvian Andes. They were experienced climbers, and climbed "Alpine-style," climbing the mountain in "one great push," without setting up ropes or base camps ahead of time. After dealing with a snowstorm and some dangerous climbing over powder formations, they reached the summit (about 21,000 feet) on the third day. The climb down proved to be far more difficult. Simpson fell and broke his leg badly. Yates decided to try to lower Simpson down the mountain, one 300-foot section of rope at a time. The climbers had run out of gas to melt snow, so they couldn't risk stopping as night came, and a violent snowstorm began. Their plodding, painful journey hit a snag when Yates inadvertently lowered Simpson over the edge of a cliff. In the storm, the men couldn't hear each other's cries, and, Yates, uncertain as to Simpson's position, and gradually sliding down the slope himself, decided to cut the rope that connected them, sending Simpson plummeting to certain death. Miraculously, Simpson survived the fall, and was faced with the prospect of getting off the mountain alone with no food, no water, and a broken leg. In Touching the Void, filmmaker Kevin Macdonald (One Day in September) tells their story, based on Simpson's book, using contemporary interviews with the two men, and a reenactment of their climb and descent, featuring Brendan Mackey as Simpson and Nicholas Aaron as Yates

The Taiga forest, on the edge of the Arctic, is a silent world of stunted conifers. The trees may be small but filming from the air reveals its true scale. A third of all trees on Earth grow here and during the short summer they produce enough oxygen to change the atmosphere. In California General Sherman, a giant sequoia, is the largest living thing on the planet, ten times the size of a blue whale. The oldest organisms alive are bristlecone pines. At more than 4,000 years old they pre-date the pyramids. But the baobab forests of Madagascar are perhaps the strangest of all.

One and a half million years ago, a new breed of ape-man walks the land. In southern Africa, Homo Ergaster has taken the next step to becoming human. They have long, modern looking noses, which cool air as they breathe. Their hairless bodies, with millions of tiny sweat glands, mean they don't pant anymore to control their temperature - they sweat. And, above all, they have big brains - nearly two-thirds the size of ours.

What is the universe made of? If you answered stars, planets, gas and dust, you'd be dead wrong. Thirty years ago, scientists first realized that some unknown dark substance was affecting the way galaxies moved. Today, they think there must be five times our understanding of the universe and the nature of reality itself has drastically changed over the last 100 years - and it's on the verge of another seismic shift. In a 17-mile-long tunnel buried 570 feet beneath the Franco-Swiss border, the world's largest and most powerful atom smasher, the Large Hadron Collider, is powering up. Its goal is nothing less than recreating the first instants of creation, when the universe was unimaginably hot and long-extinct forms of matter sizzled and cooled into stars, planets, and ultimately, us. These incredibly small and exotic particles hold the keys to the greatest mysteries of the universe. What we find could validate our long-held theories about how the world works and what we are made of -- or, all of our notions about the essence of what is real will fall apart.