Chernobyl

At the Chernobyl nuclear power plant in Soviet Ukraine, engineers are running a late-night safety test on Reactor 4, checking whether the turbines could keep the cooling pumps running for a few seconds during a power cut. The test is delayed for hours, run by an exhausted night shift, and pushed through despite the reactor behaving strangely. Just after 1:23 a.m., an operator presses the emergency shutdown button expecting it to make the reactor safe. Instead, within seconds, the power surges to many times its normal output. Two explosions tear the reactor apart, blow its thousand-tonne roof into the sky, and expose the glowing, burning reactor core directly to the open air.

Plant staff and the deputy chief engineer insist, against all the evidence in front of them, that the reactor itself cannot have exploded — Soviet reactors, they believe, are not built that way. Firefighters are called to what they are told is an ordinary electrical fire. They climb onto rubble that is lethally radioactive, with no protective equipment and no idea of the danger, to fight flames that a garden hose cannot put out.

By morning, several of those firefighters and plant workers are already gravely ill, though no one has yet told them why. A few kilometres away, in the town of Pripyat built to house the plant's workers, ordinary life continues. Families go to work, children play outside, and nobody is warned.

Rather than warn the public, local and national officials try to manage the story. Roads are quietly blocked, but Pripyat itself is not evacuated for 36 hours — residents are told nothing, and some even walk to a nearby railway bridge to watch the strange, glowing fire in the distance. When the evacuation finally comes, residents are told they will be back within three days and should pack lightly. They never return to live in their homes again.

A scientist, Valery Legasov, and a senior party official, Boris Shcherbina, are sent to take charge, alongside Ulana Khomyuk, a nuclear physicist who begins independently piecing together what actually caused the explosion. As the scale of the disaster becomes impossible to hide, an extraordinary mobilisation begins: miners are sent to dig a tunnel beneath the reactor to prevent a second, larger explosion; and when robots fail in the radiation, human "liquidators" are sent onto the reactor roof itself, for barely more than a minute at a time, to clear radioactive graphite by hand.

Meanwhile, the world beyond the Soviet Union starts to notice. It is not a Soviet announcement that reveals the disaster, but radiation alarms going off at a nuclear plant in Sweden, hundreds of kilometres away.

At the trial of the plant's senior operators, the official version of events blames the men in the control room alone: human error, and nothing more. Legasov is expected to support that story. Instead, under pressure from Khomyuk and his own conscience, he stands up in court and explains the uncomfortable engineering truth: a flaw hidden inside the reactor's own design meant that, for a few fatal seconds, pressing the "safe" emergency button had made the explosion more likely, not less. The operators made mistakes that night — but the reactor itself had been built with a fault the state had known about and never disclosed.

Telling this truth ends Legasov's career and isolates him from his colleagues and his country. Two years to the day after the disaster, having recorded his own account of what really happened in secret, he takes his own life. His analysis of the reactor's flaw survives him, is later confirmed, and helps ensure the same design fault can never cause a second Chernobyl.

The series closes on the number of official deaths its central lie was supposed to protect, and the human cost — measured in radiation sickness, evacuated towns and a poisoned landscape — that followed from a system unwilling to admit it had been wrong.

The 36-hour delay before evacuating Pripyat, and the announcement of danger by Swedish radiation monitors rather than Soviet officials, are both accurate. The international scientific consensus (from bodies including the UN's UNSCEAR and the World Health Organization) attributes 31 deaths directly to the accident and its immediate radiation exposure: two workers killed in the explosion itself, and 28 more — mostly firefighters and plant staff — who died of acute radiation sickness in the following weeks. About 200,000 "liquidators" took part in the clean-up in 1986–87, receiving significant radiation doses.

The long-term death toll from cancers linked to the radiation released is genuinely and seriously disputed: some assessments estimate roughly 4,000 eventual deaths among the most exposed groups; other researchers, using different methods and wider populations, argue the true figure could run into the tens of thousands or more. Both the reactor design and the immediate cover-up are documented fact; the exact long-term toll is one of the few things about Chernobyl that honest scientists still argue over.

Ulana Khomyuk, the physicist who investigates alongside Legasov, never existed. Mazin has explained she is a deliberate composite, standing in for the many real Soviet scientists who took real risks to establish the truth — condensing them into one character the audience could follow. The scene of townspeople watching the fire from a railway bridge, all later dying of radiation sickness — the so-called "bridge of death" — is also dramatised well beyond what the evidence supports: it draws on real rumours about the bridge, but there is no reliable record that everyone who watched from it died, and at least one known bystander from that night lived a long, healthy life afterwards.

Valery Legasov and Boris Shcherbina were real people, and Legasov's death by suicide, two years to the day after the disaster, is real. Mazin's stated theme for the whole series is simple: "the cost of lies" — the idea that a system built on comforting falsehoods eventually forces someone to pay, in full, for the truth it refused to hear.