The Lascaux cave paintings in southern France, painted around 17,000 years ago, open this look at a planet that has not always been hospitable to the people who left them. The film traces the Snowball Earth hypothesis, the idea that Earth's surface froze over almost entirely on more than one occasion hundreds of millions of years ago, leaving oceans and continents locked under ice from pole to pole. Geologists explain how they read this history out of the rock record itself, in banded iron formations and glacial deposits found even in what were once tropical latitudes, and how such a frozen world could ever have thawed back into one capable of producing complex life. The narration moves between deep time and the more recent ice ages that shaped human prehistory, using the cave art as a marker of the climate upheaval our own ancestors lived through. It is a compact tour of how ice, more than almost anything else, has repeatedly remade the planet's surface.