Contains spoilers for this episode and earlier episodes.
Seven hours after the explosion, physicist Ulana Khomyuk detects abnormal radiation in Minsk and traces the likely source to Chernobyl. A local official dismisses her concerns, so she heads towards the plant herself. In Moscow, Valery Legasov challenges the reassuring report given to the government committee: graphite outside the building suggests that the reactor core is exposed. He travels with Boris Shcherbina, who initially treats him as an obstructive subordinate. At the plant, General Pikalov obtains a high-range radiation reading that finally breaks through the managers' denials. Shcherbina has Bryukhanov and Fomin arrested, and helicopters begin dropping sand and boron onto the burning reactor. One crashes during the operation.
Pripyat's residents are eventually evacuated by bus, told to pack for a temporary absence. Meanwhile, Lyudmilla learns that the severely injured Vasily is being transferred to Moscow. Khomyuk reaches the response team with another warning: molten reactor material could reach the water collected beneath the building and cause a steam explosion. The scientists take the danger to the government and propose draining the pools, but reaching the valves means entering flooded, contaminated passages. Shcherbina asks plant workers to volunteer and explains what is at stake when money alone fails to persuade them. Alexei Ananenko, Valeri Bezpalov and Boris Baranov step forward. The episode follows them underground, where the noise of their radiation detectors grows and their failing torches leave them in darkness.