Contains spoilers for this episode and earlier episodes.
The finale returns to Pripyat before the explosion, when families are still going about their lives and plant managers are anticipating promotions. It then moves between the night of the safety test and the later trial of Dyatlov, Bryukhanov and Fomin. Shcherbina explains the test's purpose, while Khomyuk reconstructs its postponement and the arrival of a night shift unprepared to carry it out. Dyatlov pushes the operators to continue after the reactor's power collapses. During a recess, Shcherbina admits that he once thought himself unimportant; Legasov tells him that his ability to obtain men and equipment made the response possible.
In the courtroom, Legasov explains how the reactor became unstable and why pressing AZ-5 made matters worse. Its control rods had graphite tips that initially increased the reaction under the conditions created that night. He identifies a concealed design flaw as well as the operators' dangerous decisions, admitting that his earlier account in Vienna hid the full explanation. The episode presents this confrontation as a dramatized courtroom reckoning. Afterwards, KGB chairman Charkov tells Legasov that his testimony will be suppressed and his scientific influence taken away. The ending reconnects his isolation with the tapes and suicide shown in the premiere. Closing captions describe the managers' sentences and the later changes to RBMK reactors, and identify Khomyuk as a fictional composite representing the scientists who investigated the disaster.