Henry Fountain is a science writer on the Climate desk of The New York Times. He toured the Chernobyl plant and the exclusion zone around it in 2014.

The first thing to understand about the HBO mini-series “Chernobyl,” which concludes its five-part run on Monday, is that a lot of it is made up. But here’s the second, and more important, thing: It doesn’t really matter.

The explosion and fire at Chernobyl’s Unit 4 reactor on April 26, 1986, was an extraordinarily messy and grim event, a radioactive “dirty” bomb on a scale that no one — certainly not anyone in the Soviet Union — was prepared for. It remains the worst disaster in the history of nuclear power, killing more than 30 people initially (and more in the years that followed, though the numbers are much disputed) and spreading radioactive contamination across large swaths of Soviet and European territory.

In the immediate panicked aftermath, and in the months of crisis and confusion until the completion seven months later of the concrete-and-steel sarcophagus that entombed the reactor’s lethal remains, the heroes and villains numbered in the hundreds, and the supporting cast in the hundreds of thousands.