In 536 AD the sky dimmed for the better part of two years. Chinese chronicles describe a sun that gave no shadow, Byzantine writers in Constantinople note crops failing under a dry fog, and monastic records from Ireland to Palestine tell of famine, drought and plague following close behind. Archaeologist David Keys leads the investigation into what caused it, weighing comet impact, asteroid strike and volcanic eruption against tree-ring data, ice cores and the written record left across three continents. The trail points to a massive volcanic eruption, possibly in Iceland or Central America, that threw enough ash into the atmosphere to cool the planet and disrupt harvests worldwide. The film traces the aftermath through the collapse of Justinian's ambitions in the Mediterranean, the spread of the plague that bears his name, and shifts in power as far as China and the Americas, building the case that this single climatic event helped set the political map of the medieval world.