With untold millions at risk after the Chernobyl explosion, nuclear physicist Ulana Khomyuk makes a desperate attempt to reach Valery Legasov, a leading Soviet nuclear physicist, and warn him about the threat of second explosion that could devastate the continent.

Valery Legasov and Soviet Deputy Prime Minister Boris Shcherbina consider using lunar rovers to remove radioactive debris, while Ulana Khomyuk faces government hurdles in determining the truth about the cause of the explosion.