As far as we know, there is only one place in the universe on which life has taken hold - Earth - but for how much longer will this distinction remain? Astronomers are on the brink of finding other worlds the same size as Earth and the same distance from their star. Professor Brian Cox considers what it is about our world that has made it a home and asks what ingredients are necessary to turn a tiny spec of rock in space into a living, vibrant planet.
To find out, Brian has come to one of Earth's richest and most bio-diverse territories, Mexico. He begins by diving in search of our most essential ingredient in a beautiful azure sink hole or cenote, a characteristic feature and primary water source in southern Mexico's Yucatán peninsula. He then travels aboard the spectacular Copper Canyon Railway to measure the impact that the sun has on our planet, discovering how early life had to learn to survive under its glare in ways that still survive in us all today.
At the end of the train journey, he arrives in Mexico's beautiful mountain interior where he uncovers how the relationship life has with the sun has led to one on the most astonishing of all life's inventions: photosynthesis. By turning sunlight into energy, life has tapped a seemingly endless energy source and introduces a vital ingredient to the planet's atmosphere - oxygen.
Finally, Brian visits a remote enclave high up in the pine forests of central Mexico to witness one of nature's greatest sights, the arrival of the monarch butterflies. Each year they make one of the longest migrations of all the butterfly species, over 4000 kilometres from northern Canada to Mexico, by tapping into one the most elusive, intangible and perhaps rarest planetary ingredients of them all.