This episode looks at our planet as a whole and considers the key factors that have shaped its natural history. Without freshwater there is no life on the land, while the sun dominates the lives of all animals and plants on Earth and defines their habitats. The most enchanting illustration of the sun's hold on life takes place in the Arctic spring. A mother polar bear emerges from her winter den followed by two tiny cubs. She has just two weeks to prepare them for a perilous journey across the frozen sea before it melts and they become stranded.