The story of the Undertale is about the relationship between monsters and humans.
Many, many, many, many, many years ago monsters and humans lived in peace. But eventually, war broke out between the two. They fought and fought, but the humans won. Seven humans locked up every monster underground with magic. The place where the monsters were held was called “ the Barrier”.
Years later, a human happened to find the Barrier. Her name was Chara and she got stuck in the Barrier. And like other stories, a prince her her cry and came to her rescue. The prince took her to his palace and his parents, the king and queen, raised Chara as they own child. But Chara got sick one day and was about to dies. All she wanted was flowers but it was too late. The prince tried to get her flowers but when he was spotted coming from the underground her was almost killed. Beaten, the prince died.