Original: It features an alternate reality theme, where a second nuclear disaster occurs at the Chernobyl Nuclear Power Plant in the near future and causes strange changes in the area around it.
Simplified: It features an alternate reality theme, where the second nuclear disaster happens at the Chernobyl Nuclear Power Plant in the near future and makes strange changes happen to the area around it, which create a lot of mutated animals, plants, and change the laws of Physics.