Roughly 75,000 years ago, the Toba volcano on the Indonesian island of Sumatra erupts with enough force to bury more than a million square miles of land under ash, ejecting over two hundred cubic miles of volcanic material in what geologists call a super-eruption. This film follows the scientists tracing what that eruption did to the small human population living through it, testing the theory that Toba's ash cloud and the volcanic winter it triggered pushed early humans through a genetic bottleneck, at one point reducing our species to perhaps only a few thousand survivors. Geological cores, ash deposits, and DNA evidence from living populations are laid out as the case for how close humanity came to disappearing before it ever spread across the globe. The film also treats the eruption as a warning: another Toba-scale event remains possible, and the same fragility that nearly ended prehistoric humans still applies to a species now numbering in the billions but no less dependent on a stable climate.