The season finale of Chernobyl, according to its creator, Craig Mazin, is the important episode in the series.
In it, after witnessing the human cost of the devastation caused by the catastrophic incident at the powerplant, viewers see the trial where Valery Legasov (Harris), Boris Shcherbina (Skarsgård) and Ulana Khomyuk (Watson) risk everything in a bid to expose what really happened that fateful day in April 1986.
While the event is shown in the first episode, it is not shown in considerable detail. In the finale, however, viewers learn that it was a combination of technical malfunctions and human error which caused the disaster in an episode which switches between a more detailed account of the incident itself and the subsequent trial.
If you haven't seen it yet, here's a clip released from the series finale:Mazin explained in an interview with Slate that the purpose of the final episode, where the phrase "the cost of lies" was repeated, was to draw comparisons between the disaster and the current political climate.
"Well, we are experiencing something now that I used to think was mostly just a phenomenon in a place like the Soviet Union, which is a disconnection from truth," Mazin said.
"And the emergence of a cult of personality. And a distrust and debasement of experts who don't go along with whatever the official narrative is.
"It's so upsetting, and we don't know quite how to handle it. What I want people to consider is that no matter what it is we want to believe, and no matter what story it is we want to jam the world into, the truth is the truth. If you organize your life around some political party's list of things you should believe, or an individual that you think is going to come and save you, you are disconnecting yourself from truth. And there is a price to pay."

Mazin added that lessons from the disaster could help us tackle the climate change crisis:
"We live on a planet that is under threat, and scientists are warning us, just as they did in the '70s regarding RBMK reactors in the Soviet Union. Governments are choosing to listen or not listen, and people are choosing to listen or not listen. But the truth, the globe, the thermometer, doesn't care. And the RBMK didn’t care either. It didn't matter what they wanted to do that night. It didn't matter that the fatal flaw of the RBMK reactor was a state secret. The reactor didn't care. And that’s the problem we struggle with. We are attempting to make ourselves superior to fact and we are not."

"The last thing I wanted to do was just make homework," he added, explaining why he spent the four episodes of the show focusing on the human cost of the disaster.
"I think if you see how people suffer, then and only then can you hear why. You will be far more interested to know, because you understand that there was a terrible cost. You've witnessed it. You've experienced it, and you've felt it."

He further elaborated on this decision in another interview with EW.
"My hope and my intention was that people would experience the tragedy of what Chernobyl was in every regard: a scientific tragedy, a political tragedy, an emotional and personal tragedy, all of that," he said.
"Just really feel what it did to an entire country and people, and then say, 'Okay, now that you know all of that, let's see how it actually happened, because this is how we learn to keep it from happening again'."

Mazin continued:
"And when I say 'it', I don’t mean a nuclear reactor exploding, I mean a tragedy caused by lies and neglect. And there's an additional factor that I wanted to introduce which was, for most of those men in that control room, they were innocent, and I think it's important for people to know that. They just didn't know. Even the villain in the room to some extent was kind of innocent, and that's kind of a shocking thing."
The first season of Chernobyl is available to watch on Sky Atlantic and HBO.
In addition to this, due to popular demand, Mazin made the scripts for the show available to read for free online: