Adventure Time's backstory is super serious. It takes place in a post-apocalyptic world that was caused by the Great Mushroom War. The picture above shows the damage caused to Earth from it. Throughout the series, there are many references to the nuclear war.
Jake the Dog is the other main character. He has stretchy powers that allows him to shrink, grow, look like other people
Jake's in a serious relationship with a rainbow unicorn whose name is Lady Rainicorn. She only speaks Korean.