Contains spoilers for this episode and earlier episodes.
The three workers sent beneath Reactor 4 find the valves and drain the flooded pools, completing the mission left unresolved at the end of the previous episode. Legasov and Shcherbina still fear that molten fuel could reach the groundwater, so miners led by Glukhov are brought in to excavate beneath the reactor for a proposed heat exchanger. Glukhov questions the officials about the conditions his men will face instead of accepting vague assurances. Underground, the heat becomes unbearable. Because ventilation risks bringing contaminated dust into the tunnel, the miners strip off their clothes and continue working without the fans they requested.
In Moscow's Hospital No. 6, Khomyuk questions the surviving operators about the safety test. Akimov and Toptunov insist that they pressed AZ-5, the emergency shutdown button, leaving her to explain why an action meant to stop the reactor preceded its destruction. Dyatlov remains defensive. Lyudmilla gets access to Vasily and stays close despite the ward's restrictions; his apparent improvement gives way to a horrifying deterioration. When Khomyuk discovers that Lyudmilla is pregnant, their confrontation draws attention, and the KGB arrests the scientist. Legasov secures her release, while he and Shcherbina present the government with the enormous cleanup still required. Vasily dies, and Lyudmilla watches his coffin lowered into a grave and covered with concrete. Khomyuk's investigation continues, but the men who might have answered more of her questions are dying.