Simply put, the Post-Apocalyptic sub-genre refers to a Science Fiction scenario that happens after an apocalyptic event. Usually, the world is rather dark and brutal, but it can have its moments of beauty depending on the style and aesthetics. More so than not, the world in a post-apocalyptic novel is blasted with radiation, mutation, weather that has radical effects, and can be a very dreary place to be in. This also translates in the society itself, as it can be filled with violence, mutations, mixed feelings towards technology, factions with extreme agendas and religious fanaticism.
While a Post-Apocalyptic story can take place mere moments after the event that caused this setting, usually most of them are set in a time-line that has permitted the effects of the apocalypse to settle down and for the remaining population to begin their survival and adventures. A post-apocalyptic society will have many interesting themes and motifs, that are often a parody of normal societal concepts. For example, if the apocalypse has been caused by a nuclear holocaust, then perhaps a religion will be formed around the worship of radiation, nuclear bombs, or the name of the first atomic bomb.
Most of the post-apocalyptic stories are filled with adventures, actions, intrigues and discoveries. They also tickle the fancy of those who are interested in history, by means of analogy to our real history. However, the main thematic that one can expect to like in post-apocalyptic novels is the idea of survival. Survival in a post-apocalyptic context is very different to that of real-world. You might be in real-life an expert hunter, survivalist or mountaineer and you can hold your own in the challenges of real life. But you cannot expect to know the correct choice to make when you are faced with the challenges of a post-apocalyptic environment, and neither will the character that you follow in the novel. Therefore, it will be interesting to see what the character chooses to do and how different your choice could have been.
Another point often emphasized in post-apocalyptic fiction is how society has changed, often devolved, after the event. Depending on the cause of the event, we can find dystopian governments that are filled with shadow councils, fanatical warlords and religious zealots, that seek to control everyone and everything. You might find small enclaves of scientists that which to revert the events of the apocalypse and return humanity to its former state. You will find slums filled with the dregs of society that survive by scavenging, looting and illegal trading.
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