On April 26, 1986, a massive explosion rips through a reactor during a safety test at a power plant in Ukraine, triggering the worst nuclear accident in history. Within 24 hours, a lethal radioactive cloud starts to circle the globe.

In the weeks following the meltdown, workers battle lethal radiation to clear debris and construct a protective tomb around the destroyed reactor. Amid speculation, Soviet scientists reveal what they can about the cause of the catastrophe.