Long ago, two races ruled over Earth: HUMANS and MONSTERS. One day, war broke out between them and, after a long series of battles, the humans were victorious. Seven of their greatest magicians sealed the monsters to the Underground with a magic barrier, while the humans took control of the surface. However, the barrier is not a perfect seal, and the only point of entrance to the Underground is Mount Ebott.
Here begins the story of Undertale, a heartfelt story about the importance of sparing instead of killing, and loving instead of hating.
The player begins as a small child who has, unfortunately, fallen through the barrier into the “Underground”, or realm of the monsters. The child, who’s canon name is Frisk, (but can be renamed by the player in the beginning) is then greeted by a small flower by the name of Flowey, who teaches you the basics of fighting and then tries to kill you, claiming that inn this world it’s “kill or be killed”. Before he can give you a final blow, however, a goat-like creature by the name of Toriel kills him and brings you into the Ruins, where she teaches you how to not fight, but avoid attacks by monsters and then make them happy until they leave you alone and you can spare them. She then leaves you in a hallway with a cellphone, saying she has a few “errands” to take care of. Of course, no child (or player) wants to just stand there and wait, so they decide to explore, where you encounter many small monsters, your job being to spare them instead of kill them (of course, it never comes outright and says that). You also collect many items and “G”, the currency in the Underground.
At the end of your mini adventure, you once again run into Toriel, who is carrying grocery bags in both arms. She apologizes for leaving you alone for so long and offers for you to stay with her. You accept. She gives you a spare room in her home and tells you she’s made cinnamon and butterscotch pie for you, then leaves you to explore the house for yourself. If you try to explore the basement, however, Toriel will stop you, saying “there is nothing down here for you” and bringing you back upstairs.
Now, you must be making some theories by now, and I’ll tell you this: it’s not what you thought think.
Eventually, you ask if you could explore the outside world, and Toriel says she “has to do something”.
You follow her into the basement, where the door tot the outside world of monsters lies, and she claims she’s going to destroy it, because she “doesn’t want to loose another one”.
You ask her to leave, and she declines, eventually she agrees to let you go if you can defeat her in a fight. Of course, you are to spare her and dodge her attacks.
Eventually, she gives up and tells you she will let you go because she loves you, and your feels-o-meter explodes.
You walk through the door and your in a forest. Undertale has only just begun.